Get rid of dataplane delay functionality. Makes reasoning about dataplane
permit defaults too hard.

Should eventually add back in.
Make DataplanePermits the default, and DataplaneDrops the exeption.

Implement DataplaneDrops by tracking a window of where we expect them to
occur. If the dp_event occurs within that window drop. Otherwise, default to
dataplane permit.
Raise an exception if we've waited too long (violated causality)

Should ultimately replay the prefix, as we say in the paper.
add missing import
Update hassel-c to latest version
halt_on_failure in CheckInvariants should allow us to switch to interactive
mode.
Put a space after the interactive prompt
Hassel-c assumes that out ports are sorted. Explains why we weren't seeing any
output!