Better interoperation between Json::Value and JsonObject.

* Generic functions to add entries to both object models.
* Add Json::Value into JsonObjects.
* Write Json::Value to string incrementally.
Allow the Ledger to generically output to both Json models.
[fold] handlers/Ledger.cpp
New generic Ledger RPC handler.
Send output incrementally in StreamHandlerImp.
Refactor away RPCHandler::doRpcCommand