* Improve logging and error handling

This overhauls the logging and error responses from the *Conn. Namely, it
changes the logging backend to `log/slog` which makes it easier to manage the
library's logging centrally (however the old functionally to provide a custom
logger is retained). It also captures the reason why a specific connection to a
ZK server was closed in the error that is propagated to all callers. This can be
checked with `errors.Is`.

This also adds an exponential backoff feature to the client, which will make it
back off when reconnecting to a ZK server.

* Remove logger option, it messes with the logging

* Change default behavior of DNSHostProvider

* Split implementation of HostProvider, fix up tests