This serie of patches aims to reduce the usage of the
AioContexlock in block-copy, by introducing smaller granularity
locks thus on making the block layer thread safe.
This serie depends on my previous serie that brings thread safety
to the smaller API used by block-copy, like ratelimit, progressmeter
abd co-shared-resource.
What's missing for block-copy to be fully thread-safe is fixing
the CoSleep API to allow cross-thread sleep and wakeup.
Paolo is working on it.
Patch 1 provides a small refactoring, patch 2 introduces the
.method field in BlockCopyState, to be used instead of .use_copy_range,
.copy_size and .zeros.
Patch 3 provide a refactoring in preparation to
the lock added in patch 4 on BlockCopyTask, BlockCopyCallState and
BlockCopyState. Patch 5 uses load_acquire/store_release to make sure
BlockCopyCallState OUT fields are updated before finished is set to
true.