Introducing "recovering" region state in AM to mark a region in recovering status used in distributedLogReplay

There are two advantages to have this new "recovering" state in Assignment Manager for a region:

1) Instead of mark a region "recovering" in ZK, we can consolidate all region states in one place and be aware by assignment manager
2) When handing disabling table, we have to have this new state so that regions of a disabling table can be transitioned into this state for recovering.

Notes:
In the initial release of distributed log replay, we may not do this subtask for simplifications. Without the new state, we still need to create recovered edits files for regions of a disabling table. With the new state, we can retire recover edits files creation business totally.