Add a TODO to handle collisions in the UDP address cache.

The UDP address only has 256 buckets and will blissfully ignore
collisions. As a result, it is possible to get in situations (especially
when talking to lots of nodes at the same time) where we would see
excessive cache thrashing.
Add a TODO to stop doing rate-throttling on UDP address cache misses.

Currently, if we get more than one UDP address cache miss per second, we
start dropping packets. This is likely to result in unacceptable packet
loss when talking to lots of nodes.