Support for Universal mode for causal_reads in SmartRouter

Currently SmartRouter's causal reads only supports the local mode, i.e. only the writes of the session itself are considered when deciding where to send a read.

MXS-7628 adds support for global mode, i.e. the writes of all sessions are considered when deciding.

Universal mode is as global except that the GTID is read directly from the MariaDB server. I.e. not just writes through MaxScale are considered, but all writes to the server.