confmon: Check if last time was 0 before issuing throttle

On OS X, it's possible for the gethrtime function to return a very low
value (since the value itself is initialized as a diff) - this could
cause confmon to mistake this as being within a throttle. This bug was
introduced when the mach_absolute_time() functionality was reenabled.

Change-Id: Ic2aa3084052d9d992e6f93f9eaf81b5d5f1b7c30
Reviewed-on: http://review.couchbase.org/44207
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>