Consider changing /reserve, /unreserve endpoint syntax.

Right now, the "reserve" and "unreserve" endpoints work as follows:

1. slaveID and resources are sent as two separate key-value pairs in the body of the request, which is URL-encoded (application/x-www-form-urlencoded media type).
2. resources is itself a JSON document.

This is a bit weird: it would more idiomatic to send a single JSON document (using content-type application/json) with the required information. [ Updated: the initial bug description claimed that slaveID and resources were sent as query parameters, but that is incorrect. ]

Obviously, there are some backward compatibility concerns if we want to change this.