reducing the overhead of frequent table locks, v4

I didn't get a lot of comments on my the previous version of my patch
to accelerate table locks.

http://archives.postgresql.org/pgsql-hackers/2011-06/msg00953.php

Here's a new version anyway.  In this version, I have:

1. Made pg_locks show fast-path locks as well as regular locks, with a
new Boolean column to distinguish.
2. Renamed fpLWLock to backendLock, on the belief that we're going to
want something like this for quite a bit more than just this one
patch.
3. Removed some debugging code.

-- 
Robert Haas
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