Create improved decommission monitor implementation

In HDFS-13157, we discovered a series of problems with the current decommission monitor implementation, such as:

• Blocks are replicated sequentially disk by disk and node by node, and hence the load is not spread well across the cluster
• Adding a node for decommission can cause the namenode write lock to be held for a long time.
• Decommissioning nodes floods the replication queue and under replicated blocks from a future node or disk failure may way for a long time before they are replicated.
• Blocks pending replication are checked many times under a write lock before they are sufficiently replicate, wasting resources

In this Jira I propose to create a new implementation of the decommission monitor that resolves these issues. As it will be difficult to prove one implementation is better than another, the new implementation can be enabled or disabled giving the option of the existing implementation or the new one.

I will attach a pdf with some more details on the design and then a version 1 patch shortly.