We previously `join`ed on piping data from both sides, meaning
that the future didn't complete until **both** sides had disconnected.
Even if the client disconnected, it was possible the server never knew,
and we "leaked" this future.
To fix this, the `join` is replaced with a `Duplex` future, which pipes
from both ends into the other, while also detecting when one side shuts
down. When a side does shutdown, a write shutdown is forwarded to the
other side, to allow draining to occur for deployments that half-close
sockets.
Closes#434
0effefa5d7