Clean up a number of autovacuum loose ends. Make the stats collector

Clean up a number of autovacuum loose ends.  Make the stats collector
track shared relations in a separate hashtable, so that operations done
from different databases are counted correctly.  Add proper support for
anti-XID-wraparound vacuuming, even in databases that are never connected
to and so have no stats entries.  Miscellaneous other bug fixes.
Alvaro Herrera, some additional fixes by Tom Lane.