Major Description
When a new VB comes in, backfillRequestHandler will diff the VBs and raise backfill request for the brand new VBs. This should guarantee that there won't be missing data, but this isn't ideal because it will generate traffic.
However, when a node takes over a VB, XDCR needs to ensure that there's no pending backfills. Only if it is sure 100% that there's no pending backfill, can it skip raising a complete backfill request.
This is tricky because the previous VB owner node could be down (i.e. previous VB master) and no way to check. Also, periodicPush from the previous master node may not have happened (i.e. push a mid-way backfill task)
Need to think more about this.