We discovered two cases where edge connections can stall during testing:
  1. Due to final data sitting in the edge inbuf when it was resumed
  2. Due to flag synchronization between the token bucket and XON/XOFF

The first issue has always existed in C-Tor, but we were able to tickle it
in scp testing. If the last data from the protocol is able to fit in the
inbuf, but not large enough to send, if an XOFF or connection block comes in
at exactly that point, when the edge connection resumes, there will be no
data to read from the socket, but the inbuf can just sit there, never
draining.

We noticed the second issue along the way to finding the first. It seems
wrong, but it didn't seem to affect anything in practice.

These are extremely rare in normal operation, but with conflux, XON/XOFF
activity is more common, so we hit these.

Signed-off-by: David Goulet <dgoulet@torproject.org>