[MERGE] Add new LockTimeout exception

[Martin Pool]

While working on <https://bugs.launchpad.net/bzr/+bug/172392> Jono and I made
some cleanups to how locks are handled.  It turned out that bug did not need to
be fixed in bzrlib, but I think they are still worth merging.

One is that at present we do not distinguish "waited to get a lock and timed
out" from "the lock was held by someone else"; doing so would be helpful both
in testing locks and in cases like the smart server where we don't want to wait.
This adds the exception and cleans it up in some cases.  

I have a followon branch that updates more of tests and fixes some timing dependencies
in testing.

Also, the bzr serve command was redundantly disabling waiting for locks.
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