Add a small cache of locks owned by a resource owner in ResourceOwner.

Back-patch 9.3-era commit eeb6f37d, to
improve the older branches' ability to cope with pg_dump dumping a large
number of tables.

I back-patched into 9.2 and 9.1, but not 9.0 as it would have required a
significant amount of refactoring, thus negating the argument that this
is by-now-well-tested code.

Jeff Janes, reviewed by Amit Kapila and Heikki Linnakangas.