Stop the timer when shuttingdown the fetch context

Stop the timer when shuttingdown the fetch context

When canceling the last fetch, we also need to stop the fctx_expired
timer from possibly firing between the fctx_shutdown() call and the
fetch being actually destroyed along with the timer.  As there are
multiple places where fctx_shutdown() is being called without stopping
the timer, move the fctx_stoptimer() to fctx_shutdown() and cleanup the
explicit usage.