Send output incrementally in StreamHandlerImp.
* Use a generic output function to accumulate data.
* Chunk content output if necessary.
* Clean JSONRPC and remove unused code.

Hide coroutines in a compilation unit.

* Works with Boost 1.55 and up.
* Respond to review comments.
Write Json::Value using the incremental Json writer.
Allow the Ledger to generically write to either Json model.
Make RPC::Ledger generic.
Make two ErrorCode functions generic.
fixup! Make RPC::Ledger generic.
Refactor away RPCHandler::doRpcCommand
RPCHandler::doCommand becomes free function
RPC::doCommand no longer returns a value.
Adjust old handler's signature to correspond to the new handler.
[fold] remove unused variable in Ledger.h
RPC::doCommand is now fully generic.
RPC handling now returns Status.
Make sure that handlers always return Json::objectValue.
Get rid of explicit template instantiation
Extract out fillHandler in RPCHandler.
Object::Root now owns its Writer.
New RPC::executeRPC function handles string output.
fixup! New RPC::executeRPC function handles string output.
Fix handling of floating point integers.