[MERGE] add lock hooks

[Martin Pool]

Here is an updated version of this patch.  Compared to the previous one.

* New base class PhysicalLock, with a .hooks attribute, rather than having
  it in the module namespace (where it might easily be incorrectly
  imported.)

* Hooks base class gains clone() and clear_hooks() methods, which I think
  are a cleaner way to save/clear/restore the hooks in tests or similar.

* It's updated for changes in the hooks api since it was written.

* Hooks renamed to lock_acquired and lock_released in keeping with the
general style of naming.

-- 
Martin
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