Introduce "Replica", "Cache", and "Dist" semantics

Each node now has a "Cache" and "Disk" layer (though we don't actually
save anything to the disk, they exist as independent domains).

Each vBucket now has a configurable amount of replicas plus a master.

Change-Id: I3172bed02ac6d5635aa90d0200096c8465f966b2
Reviewed-on: http://review.couchbase.org/27711
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>