Implement SYN and RST counters

Currently the NetFlow/IPFIX standard defines %TCP_FLAGS, %CLIENT_TCP_FLAGS, %SERVER_TCP_FLAGS that do not contain an indication on how many relevant (e.g. SYN/RST) flags have been observed but just their presence.

This is to request the implementation of new IEs that are able to report their number (up to an upper limit, 8 bit will be enough) to the collector.