Do not reset thresholdLastCheck on each iteration

Change-Id: I3d28ad5f8014cb46f1d3e9bd5e2beb8bde9add8a
Reviewed-on: http://review.couchbase.org/41740
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>