MB-8224: Check bgFetchQueue size before scheduling vkey bgfetch

It's possible that the bgfetch can complete in the other thread
before we do this check and cause the assertion to fail.

Change-Id: Ieda14a787df63a482ed1dd203d78a57aa6a2e5cc
Reviewed-on: http://review.couchbase.org/26207
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>