Unifying Client Requests with Server style

This PR is a refactor of the client in preparation for the start of implementation of bootstrap.  I wanted to unify the style of the requests so they mimic those of the server both in use (e.g., LeshanClient.send(request)) and design (visitor and builder patterns, many pulling heavily from the stuff on the server).
+1.  That was an artifact of the original source of this code, the LwM2mResponseBuilder, that was generically typed against ClientResponse.  Since the client only has one type of response for now it is unnecessary.