Revert "Break a few abstractions, get underlying rat from SenderGang"

This reverts commit 539bfa8a9bbfa35fa7aa9aad1a0b979e730427de.

(We don't need to use a SenderGang, so we don't need these methods
for now.)
Revert "Add a version of send that doesn't need a NextHop template argument."

This reverts commit 9c6784335ac6479f18615d8a8170a58a5a4a41b1.

(We can maintain this abstraction by giving the socket an accept() method,
by Keith's suggestion.)