The following changes were made:

• virtualuser auth plugin
• scram authorization.
• an authorization plugin can authorize a client at the lowest level.
• so, the startupmessage, if necessary, will be called from the plugin, not before.
• and target: has been moved to the plugin level in the yaml config.
• pggateway connects to a remote server only after checking whitelist the database name and successful authorization on the client side.
• auth plugin config as Go struct via json marshaling/unmarshaling.
• Yaml parsing lib updated up to v3.

fixes #1