Merging async-call branch changes to HEAD:

Merging async-call branch changes to HEAD:

        Removed class fdc_t. After CommCallbackData removal, fdc_t was
        only used to check that a FD is "active" and to check that a FD
        is half_closed.  fd_table flags.open seems to be identical to
        the "active" state flag so we use that now, via newly added
        isOpen() static function. AbortChecker already maintains
        half_closed status.

        Synced ipc code to call fd_open instead of fdc_open.