new GUC var: autovacuum_process_all_tables

Hi,

Right now, when autovacuum is turned on we always assume it's supposed
to process all tables except those that have autovacuum_enabled=false.

Now, sometimes it might make more sense to keep it enabled but have it
only check for certain tables, and leave the majority of them disabled.
For this we'd have a separate GUC parameter, as in $SUBJECT (I'm not
wedded to the name), and have the user set autovacuum_enabled=true via
reloptions to enable it.

Opinions?

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support