Graceful shutdown of workers

Currently. workers are stopped by being killed. This doesn't allow any guarantees to be made as to the completion of transactions and/or observers that are currently running on each worker.

From conversations in #252, there are 3 main stop options that we should provide. The first option reflects the current behavior.
- Stop immediately (kill)
- Stop after in-progress transactions complete
- Stop after all notifications have been processed

Start by tracing how workers are currently stopped and where in-progress transactions and observers are handled. Compare the three requested stop modes—immediate kill, after transactions complete, and after notifications are processed—and identify the worker-control entry points involved. Done means each mode provides the stated completion guarantee.

Written by the indexing model from the issue text.