Log encoder not picked up from config file

[dveeden]

Bug Report

Config:

[log]
encoder = "json"

[log.log-file]
filename = "/tmp/tiproxy.log"

• This doesn't lead to the config entries being in JSON format
• Changing the cfg.Log.Encoder in NewConfig() does change the encoder.
• Some logging might be printed before the logging is fully configured, leading to wrong formats and logging to stdout instead of the configured file.

Somewhat related: #854