Optimize offchain worker api by re-using http-client (#6454)

* Fix typo in offchain's docs

* Use Self keyword in AsyncApi::new()

* Move httpclient to be part of OffchainWorkers to optimize block import

* Fix compilation errors for tests

* Add wrapper struct for HyperClient

* Use lazy_static share SharedClient amongst OffchainWorkers. Remove the need to raise the fd limit

* Revert "Use lazy_static share SharedClient amongst OffchainWorkers. Remove the need to raise the fd limit"

This reverts commit 7af97498a2383b5d7405e27823db8fd97245da41.

* Add lazy_static for tests

substrate/client/offchain/src/api/http_dummy.rs

@@ -19,8 +19,18 @@
 use sp_core::offchain::{HttpRequestId, Timestamp, HttpRequestStatus, HttpError};
 use std::{future::Future, pin::Pin, task::Context, task::Poll};
 
+/// Wrapper struct (wrapping nothing in case of http_dummy) used for keeping the hyper_rustls client running.
+#[derive(Clone)]
+pub struct SharedClient;
+
+impl SharedClient {
+	pub fn new() -> Self {
+		Self
+	}
+}
+
 /// Creates a pair of [`HttpApi`] and [`HttpWorker`].
-pub fn http() -> (HttpApi, HttpWorker) {
+pub fn http(_: SharedClient) -> (HttpApi, HttpWorker) {
 	(HttpApi, HttpWorker)
 }