Move messaging layer functionality to ssl_msg.c

Broadly, the file `ssl_tls.c` functionality from two categories:

1) An implementation of the TLS and DTLS messaging layer, that is,
   the record layer as well as the DTLS retransmission state machine.

2) Handshake parsing and writing functions shared between client and
   server (functions specific to either client or server are implemented
   in ssl_cli.c and ssl_srv.c, respectively).

This commit is a first step towards separating those functionalities
by moving a number of messaging layer related functions from ssl_tls.c
to the newly created ssl_msg.c.

Because of implementation-specific dependencies between handshake logic
and messaging layer, a number of internal SSL functions are currently
used from both the handshake and the messaging layer. Those functions
will need to made public internal (i.e. declared in the mbedtls_ name-
space in ssl_internal.h), but this is left for the next commit in order
to ease review through `git diff --color-moved`. Also, the analysis of
whether their use can/should be restricted to either ssl_tls.c or ssl_msg.c
is left for future commits.

include/mbedtls/ssl_internal.h

@@ -1063,4 +1063,16 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
                              mbedtls_ssl_transform *transform,
                              mbedtls_record *rec );
 
+/* Length of the "epoch" field in the record header */
+static inline size_t ssl_ep_len( const mbedtls_ssl_context *ssl )
+{
+#if defined(MBEDTLS_SSL_PROTO_DTLS)
+    if( ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM )
+        return( 2 );
+#else
+    ((void) ssl);
+#endif
+    return( 0 );
+}
+
 #endif /* ssl_internal.h */