crashes: eliminate crashes due to disconnecte switches

Fixes problems in the logical threads for packet out sending and policy
updating. There may still be more issues in the event handling thread.
crashes: remove debug output

This is already displayed at a separate layer.
disable fallthrough optimization

The optimization as implemented is not correct. Example to follow.
WIP
crashes: compile native binary for katnetic

This alleviates many of the crash/hang issues. It also has a very
obvious affect on convergence of the network forwarding state on
startup. Specifically, there's no packet loss for tree,2,3.
crashes: simplify error handling in send_pkt_out
crashes: less log flushing

Consistent updates flushes the log a lot. Remove the flush calls from
the per-switch operations but leave the top-level flush calls in the
implement_policy function so that the log does get flushed before each
stage of the consistent update process.
crashes: handle failures in PerPacketConsistent.bring_up_switch

By the time a SwitchUp event gets handled, the switch may have already
disconnected. Instead of crashing, print out the error and continue on.
crashes: consistent updates can handle a switch disconnect
Merge pull request #318 from frenetic-lang/crashes

stability- and performace-related changes