COM_CHANGE_USER between different authenticators fail if done right after login

If client performs COM_CHANGE_USER quickly after login the MaxScale backend protocol may use the wrong authenticator module if the COM_CHANGE_USER changes authenticator.

This happens because client protocol completes COM_CHANGE_USER and modifies session-global authentication data (new username & pw) while backend is still initiating tcp connection to server. The backend authenticator module (m_authenticator-field) has already been generated and is for the original module. Next, backend protocol will generate handshake response with the new username & pw, and these only work with the new authenticator module.

To fix this, backend protocol will need to "save" authentication data when generating m_authenticator so that the two are always in sync. How this is exactly implemented is left for later.