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.
The Avro instance is now created inside a static class method. This brings
it in line with how other modules create instances.
Converted all strings to std::string and updated their usage.
The instance and session objects are now C++ structs. The next pointers
for the sessions was removed as it is not the appropriate place to store
this information. This means that the client notification functionality is
broken in this commit.
