Use a single ticket state instead of proposed/current members.

Sigh. Sadly another big rewrite - but that's unavoidable, the
whole state logic depends on that data format.

Let's hope that it works better this time; at least, now it's
unambigous which member holds which value.
(Previously, did current->prev_ballot have an older/same/newer
value compared to proposed->ballot? Etc.)

Should be a bit more resistant against delayed messages; more
conditions when receiving messages might help.

Conflicts:
src/config.h