The functions that handle Backend classes are now methods of the class
itself.
Prefix all member variables with `m_` to distinct them from other
variables.
Changed the backend_ref_t struct into a Backend class. Replaced static
arrays with lists. Altered functions to make the code compile.
Further refactoring is needed, a part of the functions should be moved
into this new class.
Configuration errors can be resolved before the instance is created. This
avoids the unnecessary throws that were generated when an error occured.
As the configuration is stored in the router, the router sessions can use
a pointer to it instead of copying it locally. This should avoid some
unnecessary copying if more complex configuration parameters are added.
The use of a pointer instead of a reference conveys the message that the
lifetime of the object being pointed can, and most likely will, exceed the
lifetime of the function.
Also shuffled the member variables and internal functions around and
removed unneeded function declarations.
