[Virtio-fs] [virtiofsd] MR opened: Handle FUSE_INIT and FUSE_DESTROY non-concurrently

[virtiofs-bot@sinrega.org]

- Replace `futures::executor::ThreadPool` by `tokio::runtime::Runtime`.
  This prepares for using tokio's implementation of `RwLock` with
  owning lock guards that are `Send`, i.e., can be moved into other
  threads.

- Handling FUSE_INIT or FUSE_DESTROY requests shouldn't be done
  concurrently with handling some other request.  To enforce this, we
  require an exclusive reference (`&mut self`) when calling `init()`
  or `destroy()` functions.

  Furthermore, those requests (FUSE_INIT, FUSE_DESTROY) shouldn't be
  reordered with other requests.  To enforce this, we lock an `RwLock`
  from the main thread and move the lock guard into the worker thread.
  Locking from the main thread ensures that all previously queued
  requests have already acquired their lock.  This way, those requests
  that require a write-lock (FUSE_INIT, FUSE_DESTROY) cannot be
  reordered with requests that were enqueued earlier.  For similar
  reasons, they also cannot be reordered with requests that will be
  enqueued later.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/85