Run closehandle_cb on run queue instead of async queue

Instead of using isc_async_run() when closing StreamDNS handle, add
isc_job_t member to the isc_nmhandle_t structure and use isc_job_run()
to avoid allocation/deallocation on the StreamDNS hot-path.

lib/isc/netmgr/netmgr-int.h

@@ -245,6 +245,8 @@ struct isc_nmhandle {
 	LINK(isc_nmhandle_t) active_link;
 	LINK(isc_nmhandle_t) inactive_link;
 	void *opaque;
+
+	isc_job_t job;
 };
 
 typedef union {