MB-11506 Add a stat field for total sequence number difference

This stat field displays total sequence number difference
between KV store sequence number and View index sequence number
summed over all vbuckets. This gives an idea of how much indexing
is lagging behind w.r.t KV store.

Vbucket partitions that are indexible are considered and other
partitions are ignored

Change-Id: Ie810edcbf2a45617307c3d7762bb6ab492a55aaa
Reviewed-on: http://review.couchbase.org/38793
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>