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.

lib/isc/include/isc/async.h

@@ -45,4 +45,10 @@ isc_async_run(isc_loop_t *loop, isc_job_cb cb, void *cbarg);
  *\li	'cbarg' is passed to the 'cb' as the only argument, may be NULL
  */
 
+#define isc_async_current(loopmgr, cb, cbarg) \
+	isc_async_run(isc_loop_current(loopmgr), cb, cbarg)
+/*%<
+ * Helper macro to run the job on the current loop
+ */
+
 ISC_LANG_ENDDECLS