In #26831 we're preventing unexpected node RPCs by ensuring that the volume
watcher only unpublishes when allocations are client-terminal.
To mitigate any remaining similar issues, add serialization of node plugin RPCs,
as we did for controller plugin RPCs in #17996 and as recommended ("SHOULD") by
the CSI specification. Here we can do per-volume serialization rather than
per-plugin serialization.
Reorder the methods of the `volumeManager` in the client so that each interface
method and its directly-associated helper methods read from top-to-bottom,
instead of a mix of directions.
Ref: https://github.com/hashicorp/nomad/pull/17996
Ref: https://github.com/hashicorp/nomad/pull/26831
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