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)