Cleaned up various parts of the resource, request and response class
headers.
Moved `using` declarations into .cc files.
Made the Resource class non-copyable as it isn't really meant to be
copied.
The resources are now declared by simply typing the path that they match
and providing a callback for that path. This makes it easier to add new
resources without having to create a new class for each added resource
endpoint. It also removes some of the pathname processing that was done
inside the child class handler functions.
The HTTP side of the REST API is better handled with an actual
library. The libmicrohttpd library provides a convenient way of handling
the HTTP traffic between the clients and MaxScale.
The Resource class is intended to be an abstraction of a resource
tree. Each node in the tree can perform actions. The tree is traversed
depth first so that deeper command paths resolve to the correct nodes.
Currently all the base resources defined in the REST API documents are
implemented in a way that they return a 200 OK response to all
requests. When the internal data can be represented as JSON, the resources
can be hooked up to functions that generate JSON.
