Problem: does not show teminate_event in animation

Solution: treat terminate as "normal" event rather than -1, and
number other events from 2 upwards. Thus when the FSM terminates
we see the terminate event displayed properly, before it ends.
Problem: method is wrongly passing event as int

Solution: use event_t properly.
Problem: mailbox functionality is too complex

The codec used a mailbox to queue external commands it could not
process in the current state. However this is complex to use and
not evidently useful.

Solution: remove this.
Problem: names for socket handlers are not ideal

Solution: improve the names to match usage in client generator.
Problem: s_event_matrix isn't used

Commit 0fbb864 removed the mailbox functionality, so the event matrix
isn't needed.

Solution: remove it.
Merge pull request #146 from hintjens/master

Part of an ongoing cleanup of server/client codecs