Restore some GUCs which are useful in maintenance or special handling

They were taken out in XL. But at least pgxc_clean requires the
xc_maintenance_mode GUC to work properly. The other GUCs are probably not very
important from XL-perspective, but have them anyways unless we have a reason to
remove them
Allow pg_stat_statements to work with XL

XL uses a post-post-analyze hook for it's internal purposes. But this overrides
any existing hooks, including that of pg_stat_statements. Setup things so that
the previous hook gets called appropriately
Remove a duplicate call to post_parse_analyze_hook()

Seems like an oversight then XL patch was created. This was causing issues when
pg_stat_statements is loaded, for example.