Add some infrastructure for contrib/pg_stat_statements.

Add a queryId field to Query and PlannedStmt.  This is not used by the
core backend, except for being copied around at appropriate times.
It's meant to allow plug-ins to track a particular query forward from
parse analysis to execution.

The queryId is intentionally not dumped into stored rules (and hence this
commit doesn't bump catversion).  You could argue that choice either way,
but it seems better that stored rule strings not have any dependency
on plug-ins that might or might not be present.

Also, add a post_parse_analyze_hook that gets invoked at the end of
parse analysis (but only for top-level analysis of complete queries,
not cases such as analyzing a domain's default-value expression).
This is mainly meant to be used to compute and assign a queryId,
but it could have other applications.

Peter Geoghegan