MB-1143 Allow switching back to chain replication when needed.

Change-Id: I9f18a8cabd5455ea4f7c4168eb8e7fc5b45f4012
Reviewed-on: http://review.couchbase.org/27331
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>