Logging output and levels should be configurable

[vetinari]

The current logging with the standard log package can spam log files in certain circumstances:
issuing a search request to a slow server with timeouts set, reuse this connection to do another search and the old search request is dumped: https://github.com/go-ldap/ldap/blob/master/conn.go#L399-L400 - the current request is then processed normally.

Setting log level to debug should get the ldap.Debug messages in the logs, anything else disables it

My preferred choice would be logrus