Reduce log message severity

[bgrainger]

The current MySqlConnectorLogLevel.Debug messages are useful for debugging issues in MySqlConnector, but not useful for most client applications. The Info level messages have some use for clients, but are also a bit verbose.

• Introduce a Trace level for logs.
• Reduce almost all Debug logging to Trace
• Reduce most Info logging to Debug; keep only Info logging that's useful for most clients to detect abnormal behaviour; there probably shouldn't be any Info messages generated just from opening a connection, executing a command, and closing it