Merge 'Coroutinize distributed loader' from Benny Halevy

Before touching any of this code for #9559,
that requires a change when loading sstables from the staging subdirectory,
simplify it using coroutines.

Closes #10609

* github.com:scylladb/scylla:
  replica: distributed_loader: reindent populate_keyspace
  replica: distributed_loader: coroutinize populate_keyspace
  replica: distributed_loader: reindent handle_sstables_pending_delete
  replica: distributed_loader: coroutinize handle_sstables_pending_delete
  replica: distributed_loader: reindent cleanup_column_family_temp_sst_dirs
  replica: distributed_loader: coroutinize cleanup_column_family_temp_sst_dirs
  replica: distributed_loader: reindent make_sstables_available
  replica: distributed_loader: coroutinize make_sstables_available
  sstable_directory: parallel_for_each_restricted: keep func alive across calls
  replica: distributed_loader: reindent reshape
  replica: distributed_loader: coroutinize reshape
  replica: distributed_loader: coroutinize reshard
  replica: distributed_loader: reindent run_resharding_jobs
  replica: distributed_loader: coroutinize run_resharding_jobs
  replica: distributed_loader: reindent distribute_reshard_jobs
  replica: distributed_loader: coroutinize distribute_reshard_jobs
  replica: distributed_loader: reindent collect_all_shared_sstables
  replica: distributed_loader: coroutinize collect_all_shared_sstables
  replica: distributed_loader: reindent process_sstable_dir
  replica: distributed_loader: coroutinize process_sstable_dir