Revert "MB-11728 Avoid blocking view group process for get_stats()"

This reverts commit 47b2944e56342180abd5d2bac21f84184d5b6e9a.
From perf tests, it is noticed that it doesn't solve the issue raised
in MB-11728 and bottleneck is moved to updater scheduler.

Change-Id: I4dda6cf6bdcc874ec02027465099e8da90efd729
Reviewed-on: http://review.couchbase.org/39598
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Nimish Gupta <nimish@couchbase.com>