Show GIDs of two-phase commit commands as constants in pg_stat_statements

This relies on the "location" field added to TransactionStmt in 31de7e6,
now applied to the "gid" field used by 2PC commands.  These commands are
now reported like:
COMMIT PREPARED $1
PREPARE TRANSACTION $1
ROLLBACK PREPARED $1

Applying constants for these commands is a huge advantage for workloads
that rely a lot on 2PC commands with different GIDs.  Some tests are
added to track the new behavior.

Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/ZMhT9kNtJJsHw6jK@paquier.xyz

contrib/pg_stat_statements/pg_stat_statements.conf

@@ -1 +1,2 @@
 shared_preload_libraries = 'pg_stat_statements'
+max_prepared_transactions = 5