Remove redundant or unused avrorouter code

The code in avrorouter that returned the current transaction was not very
useful and it can be acquired via the REST API in a more convenient
format.

The number of created sessions is tracked on the service level so there is
no need to track it in the avrorouter.

Removed declarations for functions that do not exist and moved code around
to reduce the scope.

Documentation/Release-Notes/MaxScale-2.3.0-Release-Notes.md

@@ -42,6 +42,12 @@ The use of `router_options` with avrorouter was deprecated in MaxScale 2.1. In
 MaxScale 2.3, the use of `router_options` is no longer supported and the options
 should be given as parameters instead.
 
+### `QUERY-LAST-TRANSACTION` and `QUERY-TRANSACTION` CDC commands
+
+The CDC protocol no longer accepts the `QUERY-LAST-TRANSACTION` and
+`QUERY-TRANSACTION` commands. They were removed due to the addition of the REST
+API that provides the same information in a more easy to process format.
+
 ## New Features
 
 ### Runtime Configuration of the Cache