TLS: use isc_buffer_t for send requests

This commit replaces ad-hoc code for send requests buffer management
within TLS with the one based on isc_buffer_t.

Previous version of the code was trying to use pre-allocated small
buffers to avoid extra allocations. The code would allocate a larger
dynamic buffer when needed. There is no need to have ad-hoc code for
this, as isc_buffer_t now provides this functionality internally.

Additionally to the above, the old version of the code lacked any
logic to reuse the dynamically allocated buffers. Now, as we do not
manage memory buffers, but isc_buffer_t objects, we can implement this
strategy. It can be in particular helpful for longer lasting
connections, as in this case the buffer will adjust itself to the size
of the messages being transferred. That is, it is in particular useful
for XoT, as Stream DNS happen to order send requests in such a way
that the send request will get reused.

lib/isc/netmgr/netmgr-int.h

@@ -735,7 +735,7 @@ typedef enum {
 
 typedef struct isc_nmsocket_tls_send_req {
 	isc_nmsocket_t *tlssock;
-	isc_region_t data;
+	isc_buffer_t data;
 	isc_nm_cb_t cb;
 	void *cbarg;
 	isc_nmhandle_t *handle;