MB-7467 Reset task start time after being interrupted

So that there's no re-attempt to terminate an idle isolate
if the same mapreduce context is not re-used within the
next termination period.

Change-Id: Ia9f2a3b3e17cb8abdc055913192ae450a21fd881
Reviewed-on: http://review.couchbase.org/24953
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>