pgsql: Call pgstat_drop_database during DROP DATABASE, so that any stats

Log Message:
-----------
Call pgstat_drop_database during DROP DATABASE, so that any stats file
entries for the victim database go away sooner rather than later.  We already
did the equivalent thing at the per-relation level, not sure why it's not
been done for whole databases.  With this change, pgstat_vacuum_tabstat
should usually not find anything to do; though we still need it as a backstop
in case DROPDB or TABPURGE messages get lost under load.

Modified Files:
--------------
    pgsql/src/backend/commands:
        dbcommands.c (r1.191 -> r1.192)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c.diff?r1=1.191&r2=1.192)
    pgsql/src/backend/postmaster:
        pgstat.c (r1.145 -> r1.146)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.145&r2=1.146)
    pgsql/src/include:
        pgstat.h (r1.53 -> r1.54)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h.diff?r1=1.53&r2=1.54)