1. Moved Volume to separate file
2. Created IVolume interface and implemented current behaviour in implementation of new interface — VolumeJBOD
3. Replaced all old volume usages with new VolumeJBOD. Where it is unnecessary to have JBOD — left just IVolume.
4. Removed old Volume completely
5. Moved StoragePolicy to separated files
6. Moved DiskSelector to separated files
7. Removed DiskSpaceMonitor file
This also fixes hardlink code (when one file should be sent to multiple
servers, i.e. internal_replication == false) of writeToShard() with
distributed_storage_policy (i.e. when StorageDistributed::getPath() will
path to different filesystems).
Plus also cleanup DistributedBlockOutputStream::writeToShard() a little.
After #8756 the problem with 1 thread for each (distributed table, disk)
for distributed sends became even worse (since there can be multiple
disks), so use predefined thread pool for this tasks, that can be
controlled with background_distributed_schedule_pool_size knob.
