logalloc: Shuffle code around region::impl::compact
logalloc: Shuffle code around region::impl::compact

This includes 3 small changes to facilitate next patching:
- rename region::impl::compact into compact_segment_locked
- merging former compact with compact_single_segment_locked
- moving log print and stats update into compact_segment_locked

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

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