directories: Drop seastar::async usage
directories: Drop seastar::async usage

Now the only future-able operation remained is the call to
parallel_for_each(), all the rest is non-blocking preparation,
so we can drop the seastar::async and just return the future
from parallel_for_each.

The indendation is now good, as in previous patch is was prepared
just for that.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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