Region replicas of SPLIT and MERGED regions are kept in in-memory states until restarting master

Currently, when regions are split or merged, region replicas of the regions will become CLOSED state but will not be deleted from in-memory states (e.g. AssignmentManager). These region replicas are deleted when restarting master, but I think they should be deleted during splitting and merging regions.
One problem with this is the outdated region replicas can be online. Steps to reproduce are as follows:

1. Create a table with enabling region replication.

create "test", "cf", {REGION_REPLICATION => 2}

2. Load data to the table

(0...2000).each{|i| put "test", "row#{i}", "cf:col", "val"}

3. Split the table

split "test"

4. Disable and enable the table

disable "test"
enable "test"

After doing those steps, the outdated region replica will be online.