Simplify job scheduler service locking.

Unify all locks to just one lock protecting the entire service.
There is really no need for more complicated locking -- there is
nothing in the code that can take a long time to complete.  And
having a single lock will allow various parts of the code to be
much simpler and easier to maintain.

This is just the first step of the change, switching all of the
locking to use one lock.  With this done, we can now start
simplifying the code.  For example, JobStatus no longer needs
to do any locking (or have atomic variables and such), it can
just rely on its callers holding the global service lock.

Change-Id: If026b7ef3a46a44f6802232474e629851a232a37