TLS: try to avoid allocating send request objects

This commit optimises TLS send request object allocation to enable
send request object reuse, somewhat reducing pressure on the memory
manager. It is especially helpful in the case when Stream DNS uses the
TLS implementation as the transport.

lib/isc/netmgr/netmgr-int.h

@@ -878,6 +878,7 @@ struct isc_nmsocket {
 		} state; /*%< The order of these is significant */
 		size_t nsending;
 		bool tcp_nodelay_value;
+		isc_nmsocket_tls_send_req_t *send_req; /*%< Send req to reuse */
 	} tlsstream;
 
 #if HAVE_LIBNGHTTP2