Split pgstat file in smaller pieces

We now write one file per database and one global file, instead of
having the whole thing in a single huge file.  This reduces the I/O that
must be done when partial data is required -- which is all the time,
because each process only needs information on its own database anyway.
Also, the autovacuum launcher does not need data about tables and
functions in each database; having the global stats for all DBs is
enough.

Catalog version bumped because we have a new subdir under PGDATA.

Author: Tomas Vondra.  Some rework by Álvaro
Testing by Jeff Janes
Other discussion by Heikki Linnakangas, Tom Lane.
Move ExceptionalCondition back to postgres.h

It needs to be defined in the backend even when assertions are not
enabled.  It's cleaner to put it back, than create a separate #ifdef
section in c.h.

Per trouble report from Jeff Janes
Merge branch 'master' of /home/kgrittn/pg/postgresql into matview
First cut at system views.  No docs yet.

Per request of Josh Berkus