Add upsert mode for the Vertica dialect

[sauliusvl]

trafficstars

When in upsert mode generate MERGE statements for the Vertica dialect of the form

MERGE INTO "table" AS target USING (
  SELECT ?::INT AS "id", ?::VARCHAR AS "name"
) AS incoming 
ON (target."id"=incoming."id")
WHEN MATCHED THEN UPDATE SET "name"=incoming."name","id"=incoming."id"
WHEN NOT MATCHED THEN INSERT ("name","id")
VALUES (incoming."name",incoming."id")

Unfortunately omitting the types from the SELECT statement and having non-varchar values makes Vertica error out with

com.vertica.support.exceptions.ErrorException: 
[Vertica][VJDBC](3376) ERROR: Failed to find conversion function from unknown to int

thus I had to somehow get the column type information passed to getUpsertQuery() - I'm open to suggestions as to how to do it differently (should I maybe rebase this on top of #333? or wait until it's merged?).

Casting to un-sized types since the destination table might be pre-created with differently sized types compared to the default ones, thus casting to explicit sizes might truncate values.

Note that the WHEN MATCHED branch includes key columns as advised by the Vertica documentation.

[rhauch]

@sauliusvl, thanks for submitting. Do you want to rebase this on top of the changes from #333 that were merged a few months back?

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