Improvements for dropped UDP packets

We lost proper dropped-UDP packet survival somewhere along the way, and
this patch mostly brings it back.  Main issue: A task send via TCP (eg.
a big TaskPutKey) awaits an ACK sent via UDP... which might drop, and so
the sender has to send again (but now just a UDP control packet).
Right now KVTest (at least) survives with -random_udp_drop, i.e. a 20%
drop rate of UDP packets.  It's a little slow... but it works.
Fix a few races in Jobs & TestUtil & KVTest exposed by the slow timing.