pg_stats_recovery view

Hi,

Attached is a patch thats implements a pg_stat_recovery view that
keeps counters about processed wal records. I just notice that it
still lacks documentation but i will add it during the week.

Because it tracks redo time this introduces to GetCurrentTimestamp()
calls to the redo main loop, so i add a track_recovery GUC so only the
people that wants the view has to spent time in those calls.

Probably the most controversial part of the patch will be the addition
of a new column in RmgrData that is a pointer to a new function for
*_short_desc() these functions are similar to the *_desc functions
that already exists and that is called via rm_desc but instead of
giving full details about the record being processed it just inform of
the type of the record, for example the 2 first columns will look
something like:

rmgr: XLOG
wal_record_type: xlog switch

-- 
Jaime Casanova         www.2ndQuadrant.com
Professional PostgreSQL: Soporte 24x7 y capacitación