Move multi-threading code to a library.

This patch extracts the multi-threading code from bin/swift into
swiftclient/multithreading and adds tests.  In particular, this new way
of doing it (with context managers) will prevent non-daemonic threads
from wedging the process when unexpected exceptions happen.

I enabled reporting of which lines, specifically, are not covered by
unit tests (added -m option to "coverage report" in .unittests).

This patch includes a drive-by fix for uploading a segmented file with
--use-slo when that object already exists.  A key of "name" was used
instead of "path", raising KeyError.

There's also another drive-by fix for uploading segmented objects with
--use-slo.  Commit 874e0e4427b80e1b15b74a1557b73ba9d61443ca regressed
this by removing the capturing of thread-worker results in
QueueFunctionThread.run().  This patch restores that functionality and
the feature (uploading SLO objects).

Change-Id: I0b4f677e4a734e83d1a25088d9a74f7d46384e53