This code is now spread over main and differs in cql_test_env. The PR unifies both places and makes the manager start-stop look standard
refs: #2795Closes#15375
* github.com:scylladb/scylladb:
batchlog_manager: Remove start() method
batchlog_manager: Start replay loop in constructor
main, cql_test_env: Start-stop batchlog manager in one "block"
batchlog_manager: Move shard-0 check into batchlog_replay_loop()
batchlog_manager: Fix drain() reentrability