ECUnderReplicationHandler does not consider pending adds when finding targets

ECUnderReplicationHandler finds new target datanodes where replicas of an under replicated container can be added. However, it does not exclude datanodes that are pending addition of a replica of that container from the list of targets. Ideally, if we've already queued a command to add a replica to a DN, we shouldn't add another command.

This Jira investigates why datanodes pending adds aren't excluded. Is there a reason or is this a bug?