pgsql: Make LockDatabaseObject() AcceptInvalidationMessages().

Log Message:
-----------
Make LockDatabaseObject() AcceptInvalidationMessages().

This is appropriate for the same reasons we already do it in
LockSharedObject(): things might have changed while we were waiting
for the lock.  There doesn't seem to be a live bug here at the moment,
but that's mostly because it isn't currently used for very much.

Modified Files:
--------------
    pgsql/src/backend/storage/lmgr:
        lmgr.c (r1.100 -> r1.101)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lmgr.c?r1=1.100&r2=1.101)