Now it's scattered between dist. loader and sstable directory code making the latter quite bloated. Keeping everything in distributed loader makes the sstable_directory code compact and easier to patch to support object storage backend.
Closes#12771
* github.com:scylladb/scylladb:
sstable_directory: Rename remove_input_sstables_from_reshaping()
sstable_directory: Make use of remove_sstables() helper
sstable_directory: Merge output sstables collecting methods
distributed_loader: Remove max_compaction_threshold argument from reshard()
distributed_loader: Remove compaction_manager& argument from reshard()
sstable_directory: Move the .reshard() to distributed_loader
sstable_directory: Add helper to load foreign sstable
sstable_directory: Add io-prio argument to .reshard()
sstable_directory: Move reshard() to distributed_loader.cc
distributed_loader: Remove compaction_manager& argument from reshape()
sstable_directory: Move the .reshape() to distributed loader
sstable_directory: Add helper to retrive local sstables
sstable_directory: Add io-prio argument to .reshape()
sstable_directory: Move reshape() to distributed_loader.cc