hints: add auxiliary function for scanning high level hints directory
hints: add auxiliary function for scanning high level hints directory

We scan hints directory in two places: to search for files to replay and
to search for directories to remove after resharding. The code that
translates directory name to a shard is duplicated. It is simple now, so
not a bit issue but in case it grows better have it in one place.
Message-Id: <17267ff79b0d759d6f20212e052c6df3f495752a@scylladb.com>

avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla