Basic support for async methods (#327)

* Add basic support for async methods

* Add support of async methods into ws server

* Add support of async methods to HTTP server

* Fix subscriptions method lookup

* Remove type that appeared to be unused

* Add comment on safety of unwrap in OwnedJsonRpcRequest::borrowed

* Update utils/src/server/rpc_module.rs

Co-authored-by: Niklas Adolfsson <[email protected]>

Co-authored-by: Maciej Hirsz <[email protected]>
Co-authored-by: Niklas Adolfsson <[email protected]>