The manager is in charge of updating IO bandwidth on the respective prio class. Nowadays it uses global priority-manager, but unifying sched classes effort will require it to use non-global streaming sched group. After the patch the sched class field is unused, but it's a preparation towards huge (really huge) "switch to seastar API level 7" patch
ref: #13963Closes#13997
* github.com:scylladb/scylladb:
stream_manager: Add streaming sched group copy
cql_test_env: Move sched groups initialization up
57758ec3e1