MB-7468 Stats for number of faults experienced by the server.

Change-Id: Iac687659902f95ca6d33152be99104ab321782a2
Reviewed-on: http://review.couchbase.org/23600
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>