Rebalance should use DCP stream priority to set backfill as lower than replication traffic

Currently replicateTo latency degrades significantly when Rebalance is going on (especially p99 and p99.9).

There is a new feature to set DCP stream priorities: MB-11640. Rebalance backfill phase should use it so as to lower its priority compared to replication traffic. Here is a suggestion from Dave Rigby:

• The rebalance stream (backfill in particular?) could be initially set at medium priority; increased to high later. Medium would put the backfill phase alongside GSI etc - which in fact is probably fair as they are also classed as front-end operations. When it later changes to in-memory state the priority could be restored to high.
  • Variant - we could even set backfill phaser to low priority; given it is a bulk operation which isn't latency sensitive.