Avoid using backslash in SQL string literals

[Green-Chan]

If parameter standard_conforming_strings is off, backslashes in ordinary string literals ('...') are treated as escape characters causing problems. In particular CREATE EXTENSION citus; is failing when standard_conforming_strings=off:

postgres=# CREATE EXTENSION citus;
WARNING:  nonstandard use of escape in a string literal
LINE 6: IF substring(current_Setting('server_version'), '\d+')::int ...
                                                        ^
HINT:  Use the escape string syntax for escapes, e.g., E'\r\n'.
QUERY:  
BEGIN
-- from version 12 and up we have support for tableam's if installed on pg11 we can't
-- create the objects here. Instead we rely on citus_finish_pg_upgrade to be called by the
-- user instead to add the missing objects
IF substring(current_Setting('server_version'), '\d+')::int >= 12 THEN
  EXECUTE $$
--#include "udfs/columnar_handler/10.0-1.sql"
CREATE OR REPLACE FUNCTION columnar.columnar_handler(internal)
    RETURNS table_am_handler
    LANGUAGE C
AS '$libdir/citus_columnar', 'columnar_handler';
COMMENT ON FUNCTION columnar.columnar_handler(internal)
    IS 'internal function returning the handler for columnar tables';
-- postgres 11.8 does not support the syntax for table am, also it is seemingly trying
-- to parse the upgrade file and erroring on unknown syntax.
-- normally this section would not execute on postgres 11 anyway. To trick it to pass on
-- 11.8 we wrap the statement in a plpgsql block together with an EXECUTE. This is valid
-- syntax on 11.8 and will execute correctly in 12
DO $create_table_am$
BEGIN
  EXECUTE 'CREATE ACCESS METHOD columnar TYPE TABLE HANDLER columnar.columnar_handler';
END $create_table_am$;
--#include "udfs/alter_columnar_table_set/10.0-1.sql"
CREATE OR REPLACE FUNCTION pg_catalog.alter_columnar_table_set(
    table_name regclass,
    chunk_group_row_limit int DEFAULT NULL,
    stripe_row_limit int DEFAULT NULL,
    compression name DEFAULT null,
    compression_level int DEFAULT NULL)
    RETURNS void
    LANGUAGE C
AS '$libdir/citus_columnar', 'alter_columnar_table_set';
COMMENT ON FUNCTION pg_catalog.alter_columnar_table_set(
    table_name regclass,
    chunk_group_row_limit int,
    stripe_row_limit int,
    compression name,
    compression_level int)
IS 'set one or more options on a columnar table, when set to NULL no change is made';
--#include "udfs/alter_columnar_table_reset/10.0-1.sql"
CREATE OR REPLACE FUNCTION pg_catalog.alter_columnar_table_reset(
    table_name regclass,
    chunk_group_row_limit bool DEFAULT false,
    stripe_row_limit bool DEFAULT false,
    compression bool DEFAULT false,
    compression_level bool DEFAULT false)
    RETURNS void
    LANGUAGE C
AS '$libdir/citus_columnar', 'alter_columnar_table_reset';
COMMENT ON FUNCTION pg_catalog.alter_columnar_table_reset(
    table_name regclass,
    chunk_group_row_limit bool,
    stripe_row_limit bool,
    compression bool,
    compression_level bool)
IS 'reset on or more options on a columnar table to the system defaults';
  $$;
END IF;
END
WARNING:  nonstandard use of escape in a string literal
LINE 1: substring(current_Setting('server_version'), '\d+')::int >= ...
                                                     ^
HINT:  Use the escape string syntax for escapes, e.g., E'\r\n'.
QUERY:  substring(current_Setting('server_version'), '\d+')::int >= 12
ERROR:  null value in column "objid" of relation "pg_depend" violates not-null constraint
DETAIL:  Failing row contains (2601, null, 0, 1259, 16387, 0, n).
CONTEXT:  SQL statement "INSERT INTO pg_depend
  WITH columnar_schema_members(relid) AS (
    SELECT pg_class.oid AS relid FROM pg_class
      WHERE relnamespace =
            COALESCE(
        (SELECT pg_namespace.oid FROM pg_namespace WHERE nspname = 'columnar_internal'),
        (SELECT pg_namespace.oid FROM pg_namespace WHERE nspname = 'columnar')
     )
        AND relname IN ('chunk',
                        'chunk_group',
                        'chunk_group_pkey',
                        'chunk_pkey',
                        'options',
                        'options_pkey',
                        'storageid_seq',
                        'stripe',
                        'stripe_first_row_number_idx',
                        'stripe_pkey')
  )
  SELECT -- Define a dependency edge from "columnar table access method" ..
         'pg_am'::regclass::oid as classid,
         (select oid from pg_am where amname = 'columnar') as objid,
         0 as objsubid,
         -- ... to each object that is registered to pg_class and that lives
         -- in "columnar" schema. That contains catalog tables, indexes
         -- created on them and the sequences created in "columnar" schema.
         --
         -- Given the possibility of user might have created their own objects
         -- in columnar schema, we explicitly specify list of objects that we
         -- are interested in.
         'pg_class'::regclass::oid as refclassid,
         columnar_schema_members.relid as refobjid,
         0 as refobjsubid,
         'n' as deptype
  FROM columnar_schema_members
  -- Avoid inserting duplicate entries into pg_depend.
  EXCEPT TABLE pg_depend"
PL/pgSQL function columnar_internal.columnar_ensure_am_depends_catalog() line 3 at SQL statement

This PR eliminates usages of backslashes in SQL string literals, therefore allowing using Citus when standard_conforming_strings=off.

[Green-Chan]

This PR could probably be connected to issues #6968 and #5985, but they seem too old to me to ask the authors if it helps.

[Green-Chan]

I haven't touched string literals in tests because citus is not tested with standard_conforming_strings=off anyway. But it could also be done