Replace the shard manager's pod-keyed routing table with an executor-keyed ShardLeaseState

[Aditya1404Sal]

1/7 of the ShardManager Redesign.
Resolves GOL-445

[netlify]

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[vigoo]

let's not commit such references to ongoing tickets in comments

[vigoo]

The backward compatibility is unnecessary - you can remove it to make the codebase simpler. The Golem 1.6 release is going to be breaking in many ways.

[Aditya1404Sal]

The backward compatibility is unnecessary - you can remove it to make the codebase simpler. The Golem 1.6 release is going to be breaking in many ways.

Done.

[Aditya1404Sal]

CI note: the group6 failure is oplog_processor_shard_reassignment_no_loss (5 != 6 at sharding.rs:1218). I reproduced it locally on pristine main (748d197) as well as on this branch — ~1 in 3 attempts on both, masked by #[flaky(3)] most of the time. It's the "no checkpoint" re-delivery the test's own comment describes, not related to the routing-table change. worker-tests-group1's failure (long_running_poll_loop_interrupting_and_resuming_by_second_invocation) passed locally on this branch too. Re-running the workflow.