MB-9108 add vbucket failover log

 * Add failover log entry on VB state change to active
 * Send failover log with successful stream request response
 * Check if clients need to rollback when requesting a stream
 * Stat "failovers <vbid>" shows failover log as JSON
 * Persist failover log to disk
 * Load persisted log at warmup
 * Create entry at high seq on warmup if vbucket is in active state

Change-Id: Icacaedbe0c448075f475a6e33a4b929d4292889e
Reviewed-on: http://review.couchbase.org/30539
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>