add unit test for cbagent timestamps
add unit test for cbagent timestamps

Change-Id: I9be0c771909ed7974c7e1665fb170645665ad7c1
Reviewed-on: http://review.couchbase.org/31078
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Pavel Paulau <dd88eded64e90046a680e3a6c0828ceb8fe8a0e7@gmail.com>

couchbase/cbmonitor,couchbase/cbmonitor