[beta] Backport nonces reservations (#7439)

* Reserve nonces for signing (#6834)

* Nonce future - reserve and dispatch

* Single thread nonce tests

* Track status of reserved nonces.

* Initialization of nonce reservations.

* Prospective Signer

* Fix cli tests.

* Merge pull request #7025 from paritytech/fix-nonce-reservation

Fix nonce reservation

* Fix build.

parity/cli/mod.rs

@@ -458,7 +458,7 @@ usage! {
 			"--jsonrpc-hosts=[HOSTS]",
 			"List of allowed Host header values. This option will validate the Host header sent by the browser, it is additional security against some attack vectors. Special options: \"all\", \"none\",.",
 
-			ARG arg_jsonrpc_threads: (usize) = 0usize, or |c: &Config| otry!(c.rpc).processing_threads,
+			ARG arg_jsonrpc_threads: (usize) = 4usize, or |c: &Config| otry!(c.rpc).processing_threads,
 			"--jsonrpc-threads=[THREADS]",
 			"Turn on additional processing threads in all RPC servers. Setting this to non-zero value allows parallel cpu-heavy queries execution.",
 
@@ -1461,7 +1461,7 @@ mod tests {
 			arg_jsonrpc_apis: "web3,eth,net,parity,traces,rpc,secretstore".into(),
 			arg_jsonrpc_hosts: "none".into(),
 			arg_jsonrpc_server_threads: None,
-			arg_jsonrpc_threads: 0,
+			arg_jsonrpc_threads: 4,
 
 			// WS
 			flag_no_ws: false,