Add support for KILL QUERY <id>

When a KILL QUERY <id> is executed, MaxScale must change the ID given in the statement to the corresponding connection ID on the correct backend. This can be done by creating a one-to-many relation of internal session IDs to backend server IDs.

All of the information required to implement this, namely the connection ID, is already collected by MaxScale and only the mapping of these IDs needs to be added. The problematic part is how to expose the session ID mapping in a consistent way.