ELAPI Event resolver
    
    Started working on the async processing
    and realised that I need to have a good
    copy of the event with all the fields resolved
    so this patch has some foundation for the async
    functions (module elapi_async.c) but they
    are mostly stubbed out.
    The actual code will be added down the road.
    Instead the patch focuses on the code
    introduced in elapi_resolve.c module
    and the use of the functions from it.
    It also adds the implementation of the
    high level calls that initialize ELAPI
    with the external callbacks to be used
    during async processing (elapi_log.c).