readconnroute does not increment active_operations

The readconnroute is an exception in that it doesn't use the Backend class to manage the writes. It can do this as it doesn't care whether a result is complete or not as there's next to no logic in the router after the connection has been opened. However, a side-effect of using the Backend class is that it also increments the active operation count which is what e.g. readwritesplit uses to track the load on a server. This means that if a part of the read-only traffic goes through readconnroute and another part goes through readwritesplit, the traffic is not distributed as evenly as it should be.