KV Engine Should Support File-Based Rebalance

Key-Value Engine should support "file-based rebalance". By file-based, I mean that during the rebalance when new replicas are being built on a destination node, the backfill phase of the vbucket move should be accomplished by copying the vbucket file(s) from the source node to the destination node.

We expect some reasonable vbucket rebalance time gains if we build this capability particularly for large buckets with many small items running in full-eviction mode.