New remsql version (standalone queries only) that runs over cdb2api.

[dorinhogea]

The latest version of remsql, available for standalone queries, runs the latter over cdb2api instead of the remsql custom protocol.
There are a few additions to cdb2api, which are done using SET instructions (to minimize impact);

• passing protocol version (this lets live cross-cluster connections between different fdb versions)
• passing table version (required by distributed schema change versioning)
• passing cursor id (for tracking)
• passing schema morphing (specific for remote sql, simulates covering indexes for remote schemas) Three new tunables:
• fdb_remsql_cdb2api: switches standalone queries to cdb2api protocol
• fdb_default_version: testing only, configure a node to speak a specific remsql version by default (a remote node can downgrade this to a lower version, for backward compatibility)
• fdb_version_emulate_precdbapi: testing only, configure a remote remsql node to fail to parse new SET options and behave like 8.0

NOTE: this relies on the cdb2api ability to pass rows in sqlite format, which was developed initially to support fdb push (forwarding full sql to remote nodes).