Refactor the immediate transmission feature

The original way or handling it did not cover
message fragmentation or retransmission.
Now, the messages are always appended
to the flight and sent immediately, using 
the same function as normal flight 
transmission.
Moreover, epoch handling is different for this feature,
with a possibility to perform the usual retransmission
using previous methods. 
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 97c00e2..80da3ac 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -1203,6 +1203,9 @@
 void mbedtls_ssl_recv_flight_completed( mbedtls_ssl_context *ssl );
 int mbedtls_ssl_resend( mbedtls_ssl_context *ssl );
 int mbedtls_ssl_flight_transmit( mbedtls_ssl_context *ssl );
+#if defined(MBEDTLS_IMMEDIATE_TRANSMISSION)
+void mbedtls_ssl_immediate_flight_done( mbedtls_ssl_context *ssl );
+#endif
 #endif
 
 /* Visible for testing purposes only */