Revert "Revert AMM ReduceReplicas and parallel AMMs updates (#5335)"

[fjetter]

This reverts the revert and adds another commit. The additional commit re-establishes a client+worker message send. This is misleading since the transition system on scheduler side switched to inplace dict mutations and the refactoring dropped these messages. I cannot reason yet how this leads up to our observed deadlock but it is a start.

For reference, I intended to remove this handler entirely in #5091 which outlines a subtle, yet non critical race condition. I believe it will become obsolete with #5046 as well. I will further investigate the code to look for more potential diffs but so far it look good

xref #5335 #5297

[crusaderky]

For the sake of not blocking more than what needs to, I'd suggest retiring this PR and going on with

• Reinstate: Run multiple AMMs in parallel #5339
• Reinstate: Enhance AMM docstrings #5340
• Reinstate: AMM ReduceReplicas to iterate only on replicated tasks #5341

which, put together, duplicate this PR - except that the first two can be merged in straight away.

[fjetter]

fine by me