fix(coord) Stateful set ShardManager bug fix

[amolnayak311]

Pull Request checklist

• [x] The commit(s) message(s) follows the contribution guidelines ?
• [x] Tests for the changes have been added (for bug fixes / features) ?
• [ ] Docs have been added / updated (for bug fixes / features) ?

Current behavior :

Currently shardAssignments method given a coordinator returns the list of shards the provided coordinator are assigned to. However, the method should just return just the unassigned shards and not a static list. The current implementation returns a static list which works at startup but when nodes go down, all nodes are reported as unassigned despite those nodes serving the data. This the num_active_shards metrics incorrectly reports the shards as assigned and not active.

New behavior :

The shardAssignments's implementation is now changed to return a subset of the unassigned shards the provide coordinator should serve. If there are none, an empty list if returned. The unit tests are updated accordingly.