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 7af97498.

* Add lazy_static for tests