Include server address in all logs

When viewing logs in which multiple workers (and/or nannies) are mixed into the same stream (for example, all tests in CI), it's often not possible to tell which worker produced a message.

Including the address of the current server in all log messages would make debugging easier in these cases.

This is probably rather tricky because:
1. our loggers are all per-file global variables, not associated with a particular instance
2. getting the current instance in an async context is broken: https://github.com/dask/distributed/issues/4959

One of those two things would probably need to be refactored to make this happen.