Add cache for recomputeNamespacePath().

When search_path is changed to something that was previously set, and
no invalidation happened in between, use the cached list of namespace
OIDs rather than recomputing them. This avoids syscache lookups and
ACL checks.

Important when the search_path changes frequently, such as when set in
proconfig.

An earlier version of this patch was reviewd by Nathan Bossart. This
version simplifies a few things and is safer in case of OOM.

Discussion: https://www.postgresql.org/message-id/abf4ce8804e0e05dff8c1725ae6a8ed28b7d66e0.camel%40j-davis.com
Reviewed-by: Nathan Bossart