Implements a task scheduler for resumable potentially long running tasks (#15891)

changelog.d/15891.feature

synapse/app/generic_worker.py

synapse/server.py

synapse/storage/databases/main/__init__.py

synapse/storage/databases/main/task_scheduler.py

synapse/storage/schema/__init__.py

synapse/storage/schema/main/delta/80/02_scheduled_tasks.sql

synapse/types/__init__.py

synapse/util/task_scheduler.py

tests/util/test_task_scheduler.py