Revert "MB-7597: correct the logic of updating bgfetcher global variable"

This reverts commit f6b583f3760cc1e7df85b5bf3abbc2e016a270fc.

Change-Id: Ic959fae6a1bb6cf9a2a382342518510aa9efcc4b
Reviewed-on: http://review.couchbase.org/24903
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>