handle bucket flush

Saw this hint from Aliaksey K (alk) from some related email thread...

"Any client ... can and should (as far as I'm aware) detect flush via vbucket uuid change. I.e. for well behaved DCP client it means it will work automatically. Because flush is same as data-loss-full failover followed by rebalance (i.e. when some or all vbuckets are lost during failover and has to be re-created during rebalance)."

Not sure if cbft client does that right, so this issue is a reminder to double-check.