MB-7453: use time() for long running mapreduce task termination

I.e. because previous implementation used clock which is defined to
reflect CPU time of all process threads.

It should give us more precise long running task termination.

Change-Id: I8ab51cd94a769329be212d24dda6f275287751fe
Reviewed-on: http://review.couchbase.org/23582
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-on: http://review.couchbase.org/24952