Improve pg two-phase transactions

Improve handling of two phase transaction identifiers for PostgreSQL
when the identifier is provided by the user.
As part of this change the psycopg dialect was updated to use the DBAPI
two phase transaction API instead of executing the SQL directly.

Fixes: #13229
Change-Id: If8301a7253b4a0c88e5323c9a052c3a9fa258780