[MERGE] add lock hooks

[Robert Collins]

This adds a bzrlib.lock.hooks attribute, with physical lock hooks in it.
Currently I've only connected these to LockDir, but the intent is for
any physical lock to be represented here - LockDir is enough to debug
our test suite stipple.

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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