rpc: backpressured RPC server (bump jsonrpsee 0.20) (#1313)

This is a rather big change in jsonrpsee, the major things in this bump
are:
- Server backpressure (the subscription impls are modified to deal with
that)
- Allow custom error types / return types (remove jsonrpsee::core::Error
and jsonrpee::core::CallError)
- Bug fixes (graceful shutdown in particular not used by substrate
anyway)
   - Less dependencies for the clients in particular
   - Return type requires Clone in method call responses
   - Moved to tokio channels
   - Async subscription API (not used in this PR)

Major changes in this PR:
- The subscriptions are now bounded and if subscription can't keep up
with the server it is dropped
- CLI: add parameter to configure the jsonrpc server bounded message
buffer (default is 64)
- Add our own subscription helper to deal with the unbounded streams in
substrate

The most important things in this PR to review is the added helpers
functions in `substrate/client/rpc/src/utils.rs` and the rest is pretty
much chore.

Regarding the "bounded buffer limit" it may cause the server to handle
the JSON-RPC calls
slower than before.

The message size limit is bounded by "--rpc-response-size" thus "by
default 10MB * 64 = 640MB"
but the subscription message size is not covered by this limit and could
be capped as well.

Hopefully the last release prior to 1.0, sorry in advance for a big PR

Previous attempt: https://github.com/paritytech/substrate/pull/13992

Resolves https://github.com/paritytech/polkadot-sdk/issues/748, resolves
https://github.com/paritytech/polkadot-sdk/issues/627

substrate/client/rpc/src/testing.rs

@@ -20,11 +20,50 @@
 
 use std::{future::Future, sync::Arc};
 
-use sp_core::testing::TaskExecutor;
+/// A task executor that can be used for running RPC tests.
+///
+/// Warning: the tokio runtime must be initialized before calling this.
+#[derive(Clone)]
+pub struct TokioTestExecutor(tokio::runtime::Handle);
+
+impl TokioTestExecutor {
+	/// Create a new instance of `Self`.
+	pub fn new() -> Self {
+		Self(tokio::runtime::Handle::current())
+	}
+}
+
+impl Default for TokioTestExecutor {
+	fn default() -> Self {
+		Self::new()
+	}
+}
+
+impl sp_core::traits::SpawnNamed for TokioTestExecutor {
+	fn spawn_blocking(
+		&self,
+		_name: &'static str,
+		_group: Option<&'static str>,
+		future: futures::future::BoxFuture<'static, ()>,
+	) {
+		let handle = self.0.clone();
+		self.0.spawn_blocking(move || {
+			handle.block_on(future);
+		});
+	}
+	fn spawn(
+		&self,
+		_name: &'static str,
+		_group: Option<&'static str>,
+		future: futures::future::BoxFuture<'static, ()>,
+	) {
+		self.0.spawn(future);
+	}
+}
 
 /// Executor for testing.
-pub fn test_executor() -> Arc<sp_core::testing::TaskExecutor> {
-	Arc::new(TaskExecutor::default())
+pub fn test_executor() -> Arc<TokioTestExecutor> {
+	Arc::new(TokioTestExecutor::default())
 }
 
 /// Wrap a future in a timeout a little more concisely