Improve support for did modify services

[sakuraehikaru]

For future work on top of #193 based on #144, we should think about ways to improve Bluejay's support for did modify services.

For examples:

• Expose the discover service API
  • To allow rediscovering of services that might be added back
• Automatic shut down of listens on invalidated services
  • If a service is no longer available, Bluejay should probably end any active listens associated with that service
• Test reads and writes on invalidated services, especially long running ones; I think our error handling is good enough that this is already handled without us doing anything, but it'll be nice to actually verify with evidence