Introduce different loggers for different purposes

I've recently come across this library, and am in the process of implementing some tools using it. However, I find the logging a bit too verbose for production use:

```
2021-01-29 19:32:04,061 importer.cli [INFO] server listening on ::1:8025
2021-01-29 19:32:06,811 mail.log [INFO] Available AUTH mechanisms: LOGIN(builtin) PLAIN(builtin)
2021-01-29 19:32:06,813 mail.log [INFO] Peer: ('::1', 57254, 0, 0)
2021-01-29 19:32:06,817 mail.log [INFO] ('::1', 57254, 0, 0) handling connection
2021-01-29 19:32:06,818 mail.log [INFO] ('::1', 57254, 0, 0) Data: b'lhlo [127.0.1.1]'
2021-01-29 19:32:06,820 mail.log [INFO] ('::1', 57254, 0, 0) Data: b'mail FROM:'
2021-01-29 19:32:06,821 mail.log [INFO] ('::1', 57254, 0, 0) sender: a@b.cde
2021-01-29 19:32:06,822 mail.log [INFO] ('::1', 57254, 0, 0) Data: b'rcpt TO:'
2021-01-29 19:32:06,823 mail.log [INFO] ('::1', 57254, 0, 0) recip: z@y.xwv
2021-01-29 19:32:06,824 mail.log [INFO] ('::1', 57254, 0, 0) Data: b'data'
2021-01-29 19:32:06,833 importer.cli [INFO] Handling mail
[... my own two messages ...]
2021-01-29 19:32:06,927 mail.log [INFO] ('::1', 57254, 0, 0) EOF received
2021-01-29 19:32:06,928 mail.log [INFO] Connection lost during _handle_client()
2021-01-29 19:32:06,928 mail.log [INFO] ('::1', 57254, 0, 0) connection lost
```

All I'm interested in is basically "client connected" and "client disconnected". The rest is logged in my handler anyway. However, I have no chance of filtering these messages.

In Python, we have two dimensions that can be used to filter log messages: logger name and loglevel. Now, one can argue very much about what kind of log level to choose for different types of messages. I personally think these "info" messages that print every single message sent and received would better suit a "verbose" (which doesn't exist in Python by default) or "debug" level, as the average user doesn't usually need to see them unless they're debugging.

However, I'd rather not start a discussion on log levels but suggest to use different loggers for different purposes. Instead of a `mail.log` that shall fit all purposes, one could have `mail.smtp` logger for the `SMTP` class that prints the `connection made` and `connection lost` messages, etc. See https://docs.python.org/3/library/logging.html#logger-objects and https://docs.python.org/3/library/logging.html#logging.Logger.getChild for more information on logger hierarchy.

Right now, it's not really viable to implement such a change by just adding class-specific loggers, as the majority of the logic is contained in `SMTP`. Therefore, it might make more sense to just add purpose-specific loggers like `mail.connection` or `mail.auth`.

I think this might also be a chance to refactor the (very large and complex) SMTP class, e.g., by extracting `_handle_client` into something like a `ClientHandler` to implement separation of concerns and split responsibilities better. This eliminates the need for `SMTP` to know about any of these implementations, and maybe even swap the implementations via DOI to also make testing easier. In fact, there are already some classes like `Session` into which the algorithms might be moved, but right now they're merely used to track state, whereas the business logic remains in `SMTP`. I guess this discussion should be moved into another issue, though.