GetWorkerStopChannel no longer indicates worker shutdown for Local Activities

[yuandrew]

Expected Behavior

GetWorkerStopChannel() should indicate that a worker has begun shutting down

Actual Behavior

GetWorkerStopChannel() for local activities now indicates when the local activity specific stop channel has closed. This is due to #1875.

The solution should be to also keep the worker stop channel in the Local Activity poller and have GetWorkerStopChannel() return that, while also ensuring shutdown behaves properly and the LA specific stop channel is closed at the right time.

Once this is fixed, see TODO comment linking this issue in integration tests and have Local Activity wait on that channel closure, instead of a manual time wait, to avoid potential race conditions

Steps to Reproduce the Problem

Specifications

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• Platform: