Add "EXPLAIN PLAN" to the REST-API

Only to be taken as an example, EXPLAIN ROUTE <query statement> shows the routing decision made by MaxScale - The result should display - which service the query would go through which filters would be applied and query statement after each filter is applied, if the filter affects routing decision what was decision based on filter conditionc(e.g. Database firewall filter, named filter, hint filter) and query, finally which server the query would be routed to.