Allow specific pool implementation to provide asynchronous
implementation. vm.storage.* methods will detect if given implementation
is synchronous or asynchronous and will act accordingly.
Then it's up to pool implementation how asynchronous should be achieved.
Do not force it using threads (`run_in_executor()`). But pool
implementation is free to use threads, if consider it safe in a
particular case.
This commit does not touch any pool implementation - all of them are
still synchronous.
QubesOS/qubes-issues#2256
52c3753d61