[WIP] Async operations overhaul

[andranik-s]

Improvements compared to existing implementation:

1. Easier to add new types of async requests since this implementation just calls a UA_Service in a worker thread. Especially easy if splitting a request into several operations is not required.
2. No manual request queue processing by users (no UA_Server_getAsyncOperationNonBlocking, etc.). Just call UA_Server_runAsync in a worker thread.
3. Uses condvars to wake up workers. With existing implementation worker threads need to repeatedly sleep for some period and check the request queue

Note:
It is not tested for Windows (I do not have access to it currently)

[andranik-s]

Added async history requests to demonstrate the extensibility of this approach