Using the same implementation of the Backend and SessionCommand classes in
both schemarouter and readwritesplit will prevent duplication of code.
This commit only splits the generic parts of the class to a Backend class
which the schemarouter then extends. The session commands for both routers
are similar so they require no special handling.
All routers except the binlogrouter now fully implement the JSON
diagnostic entry point. The binlogrouter needs to be handled in a separate
commit as it produces a large amount of diagnostic output.
The schemarouter now uses shared pointers. This removes the need to copy
the class.
Following changes move the member variables inside the Backend 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.
