cleanup(miner): Simplify concurrency in experimental internal miner

[arya2]

Motivation

This PR is not yet ready for review.

It should resolve the follow up work in #8136, resolve some TODOs, and simplify the concurrent logic once completed.

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Solution

• Adds an is_closed() method to WatchReceiver
• Adds awaits in mining solver tasks so only generate_block_template is responsible for waiting when the chain is changing rapidly or blocks have been submitted successfully

Testing

This will require manual testing like that in #8136

Review

This PR is not yet ready for review.

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