bench: use std::chrono
bench: use std::chrono

Change-Id: Ia8ec3ebc3751bd20b4eb8382e863a905d9b60026
Reviewed-on: http://review.couchbase.org/51060
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Mark Nunberg <dab05a0fe859c5cb93882b2a75a0d613501b07be@couchbase.com>

mnunberg/subjson,mnunberg/subjson