Re: Faster "SET search_path"

Unfortunately, adding a "SET search_path" clause to functions slows
them down. The attached patches close the performance gap
substantially.

Changes:

0001: Transform the settings in proconfig into a List for faster
processing. This is simple and speeds up any proconfig setting.

0002: Introduce CheckIdentifierString(), which is a faster version of
SplitIdentifierString() that only validates, and can be used in
check_search_path().

0003: Cache of previous search_path settings. The key is the raw
namespace_search_path string and the role OID, and it caches the
computed OID list. Changes to the search_path setting or the role can
retrieve the cached OID list as long as nothing else invalidates the
cache (changes to the temp schema or a syscache invalidation of
pg_namespace or pg_role).