Show savepoint names as constants in pg_stat_statements

Show savepoint names as constants in pg_stat_statements

In pg_stat_statements, savepoint names now show up as constants with a
parameter symbol, using as base query string the one added as a new
entry to the PGSS hash table, leading to:
RELEASE $1
ROLLBACK TO $1
SAVEPOINT $1

Applying constants to these query parts is a huge advantage for
workloads that generate randomly savepoint points, like ORMs (Django is
at the origin of this patch).  The ODBC driver is a second layer that
likes a lot savepoints, though it does not use a random naming pattern.

A "location" field is added to TransactionStmt, now set only for
savepoints.  The savepoint name is ignored by the query jumbling.  The
location can be extended to other query patterns, if required, like 2PC
commands.  Some tests are added to pg_stat_statements for all the query
patterns supported by the parser.

ROLLBACK, ROLLBACK TO SAVEPOINT and ROLLBACK TRANSACTION TO SAVEPOINT
have the same Node representation, so all these are equivalents.  The
same happens for RELEASE and RELEASE SAVEPOINT.

Author: Greg Sabino Mullane
Discussion: https://postgr.es/m/CAKAnmm+2s9PA4OaumwMJReWHk8qvJ_-g1WqxDRDAN1BSUfxyTw@mail.gmail.com