Light Client: fetch transactions/receipts by transaction hash (#6641)

* rpc: transaction/receipt requests made async

* rpc: light client fetches transaction and uncle by hash/index

* on_demand: request type for transaction index

* serve transaction index requests in light protocol

* add a test for transaction index serving

* fetch transaction and receipts by hash on light client

* fix decoding tests

* light: more lenient cost table parsing (backwards compatible)

* fix tests and warnings

* LES -> PIP

* Update provider.rs

* proper doc comments for public functions

ethcore/light/src/on_demand/mod.rs

@@ -195,6 +195,7 @@ fn guess_capabilities(requests: &[CheckedRequest]) -> Capabilities {
 				caps.serve_headers = true,
 			CheckedRequest::HeaderByHash(_, _) =>
 				caps.serve_headers = true,
+			CheckedRequest::TransactionIndex(_, _) => {} // hashes yield no info.
 			CheckedRequest::Signal(_, _) =>
 				caps.serve_headers = true,
 			CheckedRequest::Body(ref req, _) => if let Ok(ref hdr) = req.0.as_ref() {