Optimise TLS stream for small write size (>= 512 bytes)

This commit changes TLS stream behaviour in such a way, that it is now
optimised for small writes. In the case there is a need to write less
or equal to 512 bytes, we could avoid calling the memory allocator at
the expense of possibly slight increase in memory usage. In case of
larger writes, the behviour remains unchanged.

lib/isc/netmgr/netmgr-int.h

@@ -754,6 +754,7 @@ typedef struct isc_nmsocket_tls_send_req {
 	void *cbarg;
 	isc_nmhandle_t *handle;
 	bool finish;
+	uint8_t smallbuf[512];
 } isc_nmsocket_tls_send_req_t;
 
 typedef enum isc_http_request_type {