Fix Log Levels

Right now, "prod" log level is set to info: https://github.com/aws/aws-xray-daemon/blob/master/pkg/logger/log_config.go

We should configure it to "Error" level, since we state in the [docs](https://docs.aws.amazon.com/xray/latest/devguide/xray-daemon-configuration.html#xray-daemon-configuration-configfile) that it's the least verbose. This is arguably a breaking change since prod is the default log level, but we can discuss whether that is truly the case given the current behavior is incorrect.

We should also add a "silent" log level that suppresses all log messages.

Related: #62