This commit adds asynchronous services to open62541 on client side
* Asynchronous service (highlevel read/write) are available through the API that is defined in
ua_client_highlevel_async.h
* Besides the services, the connection procedure is asynchronous as well.
* A client Application that uses the asynchronous API
adds requests by calling an asynchronous service (e.g., UA_Client_sendAsyncReadRequest(), UA_Client_sendAsyncBrowseRequest() )
checks for response with UA_Client_run_iterate() repeatedly
and processes the responses with user-defined callbacks
