[merge] use os._exit

[Martin Pool]

This changes bzr to exit through os._exit when it's done.  This function
differs from sys.exit in that it does not finalize or gc any objects
that still exist.  That has two benefits:

It suppresses the intermittent failure of the smartserver 
tests that I wrote about yesterday -- python never goes into
the terrible state of modules disappearing while in use

By avoiding gc of things that are about to be destroyed we save some
time at shutdown -- about 50ms in my test, or 14% of the time to do bzr
status in a bzr tree.

-- 
Martin
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