Introduce sub-structure of ssl_handshake_params for buffering

This commit introduces a sub-structure `buffering` within
mbedtls_ssl_handshake_params that shall contain all data
related to the reassembly and/or buffering of handshake
messages.

Currently, only buffering of CCS messages is implemented,
so the only member of this struct is the previously introduced
`seen_ccs` field.

include/mbedtls/ssl_internal.h

@@ -308,8 +308,12 @@ struct mbedtls_ssl_handshake_params
     unsigned char alt_out_ctr[8];       /*!<  Alternative record epoch/counter
                                               for resending messages         */
 
-    uint8_t seen_ccs;                   /*!< Indicates if a CCS message has
+    struct
+    {
+        uint8_t seen_ccs;               /*!< Indicates if a CCS message has
                                          *   been seen in the current flight. */
+
+    } buffering;
 #endif /* MBEDTLS_SSL_PROTO_DTLS */
 
     /*