Add a pgstat config column to pg_database, so this, entire thing can be enabled/disabled on a per db basis

Hi,

Looking at the code, it seems the $SUBJECT (comment) is obsolete. Indeed, we
can set it with 'ALTER DATABASE foo SET track_*'.

One thing that bothers me is the fact that if i turn it off, do a lot of stuff
and then turn it on my counters will be wrong. :( Maybe should we call
pgstat_reset_counters() in a new-function (assign_track_counts(?) that will be
in assign_hook for guc 'track_counts')?


--  Euler Taveira de Oliveira http://www.timbira.com/