Phase II : Erasure Coding Offline Recovery & Read/Write Improvements

This is an umbrella Jira for EC offline recovery work.

As part of Phase-I, we have finished the functionality of erasure coding write and reads as part of the Jira HDDS-3816. That being stabilized in a parallel effort. 

So, this Jira to start pending recovery work to finish end-end EC MVP.

Requirements in brief: 

1. The SCM to identify the lost containers and schedule for the reconstructions.
2. DNs to start reconstructing the containers upon the request from DN.
3. We can decide whether we create new RM at SCM for EC work or we just reuse existing one. Currently there are interest to start a new RM to start it clean as the existing one already complex enough.
4. DNs to figure out the blocks them self by interacting with multiple EC block containers as single EC container may not have full set of blocks. Either first container or parity containers should have full block set. 

I am splitting the offline recovery part of design from HDDS-3816 and post here soon.

Stay tuned for the updated doc.

We will also create new branch for this work in some time