This PR is the second step in refactoring the Hinted Handoff module. It cleans up the contents of the file `hint_storage.cc`. The biggest change is the transition from continuations to coroutines.
Refs #15358Closesscylladb/scylladb#15496
* github.com:scylladb/scylladb:
db/hints: Alias segment list in hint_storage.cc
db/hints: Rename rebalance to rebalance_hints
db/hints: Clean up rebalance() in hint_storage.cc
db/hints: Coroutinize hint_storage.cc
db/hints: Clean up remove_irrelevant_shards_directories() in hint_storage.cc
db/hints: Clean up rebalance_segments() in hint_storage.cc
db/hints: Clean up rebalance_segments_for() in hint_storage.cc
db/hints: Clean up get_current_hints_segments() in hint_storage.cc
db/hints: Rename scan_for_hints_dirs to scan_shard_hint_directories
db/hints: Clean up scan_for_hints_dirs() in hint_storage.cc
db/hints: Wrap hint_storage.cc in an anonymous namespace
ecceb554c3