Allow block replication parameters to be refreshable

There are 3 key parameters that control the speed of block replication across the cluster:

dfs.namenode.replication.max-streams
dfs.namenode.replication.max-streams-hard-limit
dfs.namenode.replication.work.multiplier.per.iteration

These are used when decomissioning nodes and when under replicated blocks are being recovered across the cluster. There are times when it may be desirable to increase the speed of replication and then reduce it again (eg during off peak hours) without restarting the namenode.

Therefore it would be good to be able to reconfigure / refresh these parameters dynamically without a namenode restart.

This Jira is to allow these parameters to be refreshed at runtime without a restart.