Add Volume Health Information

[tiagolobocastro]

fix(rest-plugin): show missing target if missing

If the target is meant to be there, but it's missing, state that.

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feat(rest-plugin): add clean shutdown to volume output

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feat(rest-plugin): add replica topology health info

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feat(rest): add health information

Enhance the volume with pstor health information which is retried from
etcd using watches.

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feat(core/watcher): add pstor watcher for volume health info

Add a volume health watcher which caches updates from etcd for every volume.
When etcd entry is deleted, the equivalent entry in the cache is also deleted.

When volume is republished sucessfully, the older target health information are
also removed from the cache.

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fix: remove volume health entries on delete

When a volume is deleted, destroy all related health entries.
It has been observed that upon republished, the previous info
is not deleted, so even if we fix that a lot of entries may
already be leaked, so let's ensure we delete all entries for
a given volume on volume deletion.

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refactor(pstor/watcher): add missing value on watch callback