Flusher initializes the vbstate range based on VBucket::state

The flusher should persist the new vbstate on disk based only on the information retrieved from the disk queue.
Issue already faced and partially addressed in MB-35331.

Code at EPBucket::flushVBucket() (http://src.couchbase.org/source/xref/mad-hatter/kv_engine/engines/ep/src/ep_bucket.cc#594-599):

594                  folly::SharedMutex::ReadHolder rlh(vb->getStateLock());

595                  if (vb->getState() == vbucket_state_active) {

596                      if (maxSeqno) {

597                          range = snapshot_range_t(maxSeqno, maxSeqno);

598                      }

599                  }

Example:

1. frontend queues a set-vbstate(active) item into the CheckpointManager
2. flusher pulls that item from the CM and initializes the new vbstate to persist -> note that the flusher should initialize the range as per new-vb-state=active
3. before the flusher executes line 594, the frontend processes a new set-vbstate(replica) -> VBucket::state is set to replica immediately and a new set-vbstate(replica) items is queued into the CM
4. now the flusher executes from line 594 and skips the block, as it reads (VBucket::state = replica)