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
Merge branch 'master' of ssh://git.code.sf.net/p/postgres-xl/postgres-xl
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.
Use ResourceOwner to track internally prepared statements

XL uses prepared statements internally to execute querys on remote datanodes.
But in case of transaction aborts, these statements are left open on the
datanodes causing issues later. We now track such prepared statements using
transaction resource owner and drop them in case of aborts.
Do not read prototype config file when dealing with user specified conf file

pgxc_ctl utility was picking up parameters from the default configuration when
such parameters are not defined in the conf file. This creates all sorts of
unintentional side effects. Fix that by skipping default conf while dealing
with a specified conf file
Merge remote-tracking branch 'upstream/master'