Custom RPC implementation for node. (#3109)

* Allow RPCs to be customized.

* Implement node-rpc extensions.

* Working on a test.

* Add node-testing crate.

* Fix genesis test config

* Fix nonce lookups.

* Clean up.

* Fix expected block type.

* Make the RPC extension function optional.

* Fix service doc test.

* Bump jsonrpc.

* Bump client version.

* Update Cargo.lock

* Update jsonrpc.

* Fix build.

* Remove unused imports.

* Fix signed extra.

* Post merge clean up.

* Fix tests.

* Patch hashmap-core.

* Fix build.

* Fix build.

* Remove hashmap_core patches.

substrate/node/runtime/src/lib.rs

@@ -576,6 +576,12 @@ impl_runtime_apis! {
 		}
 	}
 
+	impl node_primitives::AccountNonceApi<Block> for Runtime {
+		fn account_nonce(account: AccountId) -> Index {
+			System::account_nonce(account)
+		}
+	}
+
 	impl consensus_primitives::ConsensusApi<Block, babe_primitives::AuthorityId> for Runtime {
 		fn authorities() -> Vec<babe_primitives::AuthorityId> {
 			Babe::authorities().into_iter().map(|(a, _)| a).collect()