Implement PIP messages, request builder, and handlers (#4945)

* return errors on database corruption

* fix tests, json tests

* fix remainder of build

* buffer flow -> request credits

* proving state backend

* generate transaction proofs from provider

* network messages for transaction proof

* transaction proof test

* test for transaction proof message

* fix call bug

* request transaction proofs from on_demand

* most of proved_execution rpc

* proved execution future

* initial request definitions

* RLP encoding and decoding for requests

* proofs of non-existance in ProvingBlockChainClient

* new requests in provider.

* encode and decode responses

* complete initial request changes

* handle request packet in LightProtocol

* handle response packets

* implement requesting from

* re-do cost table

* get tests compiling

* fix cost table RLP encoding

* roundtrip tests for request types

* request builder tests

* move request_builder -> request::builder

* get network tests working

* return only complete headers responses

* request builder improvements

* New version of jsonrpc.

* split request filling into fill,complete

* Better invalid encoding messages

* Fixing deprecated methods of tokio_core

* use PIP messages in on_demand, old API

* migrate oneshot::complete to send in on_demand

* get on_demand tests to compile

* port ethsync to PIP messages

* adjust to minor on_demand API changes in RPC

* Using dedicated branch for jsonrpc

* Bump

ethcore/light/src/net/error.rs

@@ -56,6 +56,8 @@ pub enum Error {
 	UnknownPeer,
 	/// Unsolicited response.
 	UnsolicitedResponse,
+	/// Bad back-reference in request.
+	BadBackReference,
 	/// Not a server.
 	NotServer,
 	/// Unsupported protocol version.
@@ -78,6 +80,7 @@ impl Error {
 			Error::WrongNetwork => Punishment::Disable,
 			Error::UnknownPeer => Punishment::Disconnect,
 			Error::UnsolicitedResponse => Punishment::Disable,
+			Error::BadBackReference => Punishment::Disable,
 			Error::NotServer => Punishment::Disable,
 			Error::UnsupportedProtocolVersion(_) => Punishment::Disable,
 			Error::BadProtocolVersion => Punishment::Disable,
@@ -109,6 +112,7 @@ impl fmt::Display for Error {
 			Error::WrongNetwork => write!(f, "Wrong network"),
 			Error::UnknownPeer => write!(f, "Unknown peer"),
 			Error::UnsolicitedResponse => write!(f, "Peer provided unsolicited data"),
+			Error::BadBackReference => write!(f, "Bad back-reference in request."),
 			Error::NotServer => write!(f, "Peer not a server."),
 			Error::UnsupportedProtocolVersion(pv) => write!(f, "Unsupported protocol version: {}", pv),
 			Error::BadProtocolVersion => write!(f, "Bad protocol version in handshake"),