The series fixes a couple of crashes that were found during starting and
stopping Scylla with raft while doing ddl operations. Most of them
related to shutdown order between different components.
Also in scylla-dev gleb/group0-fixes-v1
CI https://jenkins.scylladb.com/job/releng/job/Scylla-CI/749/
* origin-dev/gleb/group0-fixes-v1:
migration manager: remove unused code
db/system_distributed_keyspace: do not announce empty schema
main: stop raft before the migration manager
storage_service: do not pass the raft group manager to storage_service constructor
main: destroy the group0_client after stopping the group0
9a88bc260c