batchlog_manager: Coroutinize drain and stop
batchlog_manager: Coroutinize drain and stop

This is not identical change, if drain() resolves with exception we end
up skipping the gate closing, but since it's stop why bother

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

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