Bug 965587 - Add lock to Firefox Health Report uploading; r=rnewman

Previously, it was technically possible for the FHR client to have
multiple simultaneous uploads. While this should never occur in
well-behaving systems, server logs have indicated that this behavior
might be occurring.

This patch adds a lock around uploading to ensure only 1 upload
operation may be active at a given time.

To measure the impact of this change in the wild, we added a counter
that increments whenever a held lock is encountered.

--HG--
extra : rebase_source : f0bb5065a3618cd335b6b6f31e4e68850f31f151