Integrate transaction pool to the proposal logic (#80)

* reshuffle consensus libraries

* polkadot-useful type definitions for statement table

* begin BftService

* primary selection logic

* bft service implementation without I/O

* extract out `BlockImport` trait

* allow bft primitives to compile on wasm

* Block builder (substrate)

* take polkadot-consensus down to the core.

* test for preemption

* fix test build

* Fix wasm build

* Bulid on any block

* Test for block builder.

* Block import tests for client.

* Tidy ups

* clean up block builder instantiation

* justification verification logic

* JustifiedHeader and import

* Propert block generation for tests

* network and tablerouter trait

* use statement import to drive creation of further statements

* Fixed rpc tests

* custom error type for consensus

* create proposer

* asynchronous proposal evaluation

* inherent transactions in polkadot runtime

* fix tests to match real polkadot block constraints

* implicitly generate inherent functions

* add inherent transaction functionality to block body

* block builder logic for polkadot

* some tests for the polkadot API

* avoid redundancy in native code compatibility check

* helper for extracting nonce

* transaction pool implementation

* transaction pool

* integrate transaction pool with proposer

* indentation

* kill storage keys module

* accept new transactions to replace old

polkadot/consensus/src/error.rs

@@ -41,6 +41,13 @@ error_chain! {
 			description("Proposal had wrong parent hash."),
 			display("Proposal had wrong parent hash. Expected {:?}, got {:?}", expected, got),
 		}
+		ProposalTooLarge(size: usize) {
+			description("Proposal exceeded the maximum size."),
+			display(
+				"Proposal exceeded the maximum size of {} by {} bytes.",
+				::MAX_TRANSACTIONS_SIZE, ::MAX_TRANSACTIONS_SIZE.saturating_sub(*size)
+			),
+		}
 	}
 }