Add isc_async_current() macro to run job on current loop

Add isc_async_current() macro to run job on current loop

Previously, isc_job_run() could have been used to run the job on the
current loop and the isc_job_run() would take care of allocating and
deallocating the job.  After the change in this MR, the isc_job_run()
is more complicated to use, so we introduce the isc_async_current()
macro to suplement isc_async_run() when we need to run the job on the
current loop.