Fix race conditions (2)

[jpcima]

expected to fix #511

It was observed that QueuedFileData has a FileId member, and it's copied, therefore not RT-safe.

This implements the file queue in a different strategy, and removed problematic code at the same time.

• replace FileId Region::sampleId with a shared pointer.
  That permits to pass it in the message queue, without worrying about a corruption in case the region gets removed while the FileId is still in the queue.

• the queued file data receives a id member of type weak_ptr, meaning it auto-nulls itself if the Region gets deleted.
  In this model, the FilePool must null-check the id at relevant places to check that the request is still valid.
  Since it auto-nulls on erasing regions, we don't need emptyFileLoadingQueues, loading jobs will clean themselves.