block-copy: add CoMutex lock

Group various structures fields, to better understand what we need to
protect with a lock and what doesn't need it.
Then, add a CoMutex to protect concurrent access of block-copy
data structures. This mutex also protects .copy_bitmap, because its thread-safe
API does not prevent it from assigning two tasks to the same
bitmap region.

Exceptions to the lock:
- .sleep_state is handled in the series "coroutine: new sleep/wake API"
and thus here left as TODO.

- .finished, .cancelled and reads to .ret and .error_is_read will be
protected in the following patch, because are used also outside
coroutines.

- .skip_unallocated is atomic. Including it under the mutex would
increase the critical sections and make them also much more complex.
We can have it as atomic since it is only written from outside and
read by block-copy coroutines.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20210624072043.180494-5-eesposit@redhat.com>
  [vsementsov: fix typo in comment]
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

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