Returns next sequence number to be used.
Return the next sequence number to be used on this channel. This function must not be confused with vortex_channel_get_next_expected_seq_no.
This function is actually used to know what message seq number to use on next frame to be sent. If you what to get next frame seq no to expected to be received on this channel you must use vortex_channel_get_next_expected_seq_no.