Feature - Skipping ahead in TCP flows. (#163)

* Skipping forward in TCP streams, from an out-of-order callback.

- Added the ability to skip forward in a flow to a sequence number, with the intention of doing so in an out of order callback.
- Re-ordered Flow packet processing, to allow skipTo in out of order callback on stream start (flow sequence number is 0).
- Fixed missing seq_compare in flow code.

* Renamed skipTo to advance_sequence.

include/tins/tcp_ip/flow.h

@@ -165,6 +165,24 @@ public:
      */
     void process_packet(PDU& pdu);
 
+    /**
+     * \brief Skip forward to a sequence number
+     *
+     * This allows to recover from packet loss, if we just do not see all packets of
+     * an original stream. This recovery can only sensibly triggered from the application
+     * layer.
+     *
+     * This method is particularly useful to call from an out of order callback, if
+     * the application wants to skip forward to this out of order block. The application
+     * will then get the normal data callback!
+     *
+     * IMPORTANT: If you call this method with a sequence number that is not exactly a
+     * TCP fragment boundary, the flow will never recover from this.
+     *
+     * \param seq The sequence number to skip to.
+     */
+    void advance_sequence(uint32_t seq);
+
     /**
      * Indicates whether this flow uses IPv6 addresses
      */