Make HTTP20Adapter reuse connections.
Better GoAway logging.
Don't send WINDOWUPDATEs for empty DATA frames.

The spec mandates that the minimum size of the window increment in a
WINDOWUPDATE frame be 1. Stop sending the WINDOWUPDATE frame when we
receive an empty DATA frame.

Also, do increment the window size for the connection when data is
received, or we'll stop recieving data at all.