Fix how regular tasks that have a virtual parent are created in task_manager::module::make_task: set sequence number of a task and subscribe to module's abort source. Fixes: #21278. Needs backport to 6.2 Closes scylladb/scylladb#21280 * github.com:scylladb/scylladb: tasks: fix sequence number assignment tasks: fix abort source subscription of virtual task's child