New JsonWriter for improved client performance (RIPD-439):
When JSON-RPC and Websocket responses are calculated, the result is stored
in intermediate Json::Value objects and later composed in a single linear
memory buffer before being sent to the socket.  These classes support a
new model for building responses that supports incremental construction
of JSON replies in constant time and removes the requirement that all
data returned be located in continuguous memory.
* New JsonWriter incrementally writes JSON with O(1) granularity and memory.
* Array, Object are RAII wrappers for the O(1) JsonWriter.