Refactor async_rw_mutex

[msimberg]

This is an attempt at slightly simplifying and optimizing the internals of async_rw_mutex. This avoids the needs for a lock to keep track of continuations and instead triggers continuations through operation states. The continuations are linked to each other through an intrusive linked list of operation states. These changes also avoid the need to have a weak shared pointer between shared states.

Overall I'm hoping that the removal of extra reference counting and locks will slightly improve performance, but fundamentally the structure is still the same, requiring the same amount of dynamic allocations (cf. #1125; this PR does not address that) as before (in fact, one more allocation for the value stored by the mutex). So the impact may be minimal in terms of performance. However, I'm also making these changes to make the dependency triggering a bit more understandable.

I'm opening this for testing, but it's far from ready for merging. It still needs:

• Some words on the internals of async_rw_mutex, how are triggers continuations etc. Ideally with some diagrams as well.
• Reduce duplication of internals. Many helper functions and type definitions are duplicated for the void and non-void cases.
• Benchmarks, both in pika and DLA-Future.
• Avoid exposing the allocator in all templates?
• Relax memory orderings?
• Reshuffle class members for better access patterns?

[codacy-production]

Coverage summary from Codacy

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Coverage variation	Diff coverage
✅ -0.02% (target: -1.00%)	✅ 92.79% (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (07e3e5b)	18217	13756	75.51%
Head commit (6def689)	18227 (+10)	13759 (+3)	75.49% (-0.02%)

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Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1379)	111	103	92.79%

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