TLSDNS: call send callbacks after only the data was sent

This commit ensures that write callbacks are getting called only after
the data has been sent via the network.

Without this fix, a situation could appear when a write callback could
get called before the actual encrypted data would have been sent to
the network. Instead, it would get called right after it would have
been passed to the OpenSSL (i.e. encrypted).

Most likely, the issue does not reveal itself often because the
callback call was asynchronous, so in most cases it should have been
called after the data has been sent, but that was not guaranteed by
the code logic.

Also, this commit removes one memory allocation (netievent) from a hot
path, as there is no need to call this callback asynchronously
anymore.

lib/isc/netmgr/netmgr-int.h

@@ -948,6 +948,7 @@ struct isc_nmsocket {
 			TLS_STATE_CLOSING
 		} state;
 		isc_region_t senddata;
+		ISC_LIST(isc__nm_uvreq_t) sendreqs;
 		bool cycle;
 		isc_result_t pending_error;
 		/* List of active send requests. */