measure avg_bg_wait_time in bgfetcher tests

Change-Id: I94d7cabe6ea5983ddcfb5a7dd1347df6327d98ae
Reviewed-on: http://review.couchbase.org/37129
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: buildbot <build@couchbase.com>