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/primitives/src/lib.rs

@@ -66,3 +66,11 @@ pub type BlockId = generic::BlockId<Block>;
 
 /// Opaque, encoded, unchecked extrinsic.
 pub type UncheckedExtrinsic = OpaqueExtrinsic;
+
+client::decl_runtime_apis! {
+	/// The API to query account account nonce (aka index).
+	pub trait AccountNonceApi {
+		/// Get current account nonce of given `AccountId`.
+		fn account_nonce(account: AccountId) -> Index;
+	}
+}