main: make alternator shutdown hook-based
main: make alternator shutdown hook-based

In order to properly handle not only shutdown, but also
decommission, drain and similar operations, alternator
shutdown is now registered as a client shutdown hook,
which allows storage service to trigger its shutdown routines.

Fixes #5886

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