Handle port re-use gracefully.

Use a separate send queue to hold segments prior to transmitting them,
as well as the receive queue originally used to order them. This allows
us to immediately determine when a session is finished, and remove the
session from the lookup table so that the port can be re-used.
Make load test compute percentage of requests replicated.
Remove a few completed TODO items.
Remove an unused definition
Remove an assertion violated by simulated netem corruption.
Tweak load tests to be somewhat more realistic.
Add a few TODO items.