Store outgoing record sequence number outside record buffer

This commit is another step towards supporting the packing of
multiple records within a single datagram.

Previously, the incremental outgoing record sequence number was
statically stored within the record buffer, at its final place
within the record header. This slightly increased efficiency
as it was not necessary to copy the sequence number when writing
outgoing records.

When allowing multiple records within a single datagram, it is
necessary to allow the position of the current record within the
datagram buffer to be flexible; in particular, there is no static
address for the record sequence number field within the record header.

This commit introduces an additional field `cur_out_ctr` within
the main SSL context structure `mbedtls_ssl_context` to keep track
of the outgoing record sequence number independent of the buffer used
for the current record / datagram. Whenever a new record is written,
this sequence number is copied to the the address `out_ctr` of the
sequence number header field within the current outgoing record.

include/mbedtls/ssl.h

@@ -1116,6 +1116,8 @@ struct mbedtls_ssl_context
     size_t out_msglen;          /*!< record header: message length    */
     size_t out_left;            /*!< amount of data not yet written   */
 
+    unsigned char cur_out_ctr[8]; /*!<  Outgoing record sequence  number. */
+
 #if defined(MBEDTLS_ZLIB_SUPPORT)
     unsigned char *compress_buf;        /*!<  zlib data buffer        */
 #endif