main: Return a shared_ptr from defer_verbose_shutdown
main: Return a shared_ptr from defer_verbose_shutdown

This moves a few calls to make_shared to a single location.

This makes it easier to drop a dependency on the differences between
seastar::make_shared and std::make_shared.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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