pgsql: Clean up a number of autovacuum loose ends.

Log Message:
-----------
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.

Modified Files:
--------------
pgsql/src/backend/access/transam:
xlog.c (r1.212 -> r1.213)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.212&r2=1.213)
pgsql/src/backend/commands:
analyze.c (r1.87 -> r1.88)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/analyze.c.diff?r1=1.87&r2=1.88)
vacuum.c (r1.311 -> r1.312)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c.diff?r1=1.311&r2=1.312)
vacuumlazy.c (r1.55 -> r1.56)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c.diff?r1=1.55&r2=1.56)
pgsql/src/backend/libpq:
hba.c (r1.145 -> r1.146)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/hba.c.diff?r1=1.145&r2=1.146)
pgsql/src/backend/postmaster:
autovacuum.c (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c.diff?r1=1.1&r2=1.2)
pgstat.c (r1.101 -> r1.102)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.101&r2=1.102)
postmaster.c (r1.460 -> r1.461)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.460&r2=1.461)
pgsql/src/backend/utils/init:
postinit.c (r1.153 -> r1.154)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/postinit.c.diff?r1=1.153&r2=1.154)
pgsql/src/backend/utils/misc:
guc.c (r1.278 -> r1.279)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.278&r2=1.279)
pgsql/src/include/access:
xlog.h (r1.66 -> r1.67)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog.h.diff?r1=1.66&r2=1.67)
pgsql/src/include/libpq:
hba.h (r1.38 -> r1.39)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/hba.h.diff?r1=1.38&r2=1.39)
pgsql/src/include:
pgstat.h (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h.diff?r1=1.33&r2=1.34)