Add DataNode state and transitions for a node going through upgrade

A typical SCM sequence for driving datanodes through upgrade would be

Something like following :                                                                  

• client sends Finalize                                                      

• SCM moves to Finalizing states. If SCM crashes, and it comes up            

   it will always restart from this state.                                    

• SCM disallows new pipeline creation, SCM in safe mode                      

    (SCM Freeze for new pipeline)                                             

• SCM closes existing pieplines                                              

• SCM updates MLV = SLV if not already so. Update on-disk MLV state.         

• SCM moves all data nodes to HEALTHY_READONLY state. Please note that       

   initial state for all data node is HEALTHY_READONLY. For data nodes        

   to move from HEALTHY_READONLY -> HEALTHY, they need to send atleast        

   one heartbeat where DN.MLV == SCM.MLV                                      

• SCM waits for few heartbeats                                               

• SCM allows new pipeline creation (SCM thaw for new pipeline creation).     

   New Pipelines can be created if enough HEALTHY data nodes are found.       

• If SCM comes across any data node heart beat with DN.MLV < SCM.MLV => SCM sends

   that data node finalize command 

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As part of this, we would be introducing a new state HEALTHY-READONLY in DataNode state machine maintained in SCM .

This Jira will be used to make changes in the datanode state machine.