MB-8241: Rename ViewBtreeStates to just ViewStates

Change-Id: I9e1aa9f0d8cc86231f198c97627f4673a1f67168
Reviewed-on: http://review.couchbase.org/26124
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>