largefiles: getlfile must hit end of HTTP chunked streams to reuse connections
We did read the exactly the right number of bytes from the response body. But
if the response came in chunked encoding then that meant that the HTTP layer
still hadn't read the last 0-sized chunk and expected the app layer to read
more data from the stream. The app layer was however happy and sent another
request which had to be sent on another HTTP connection while the old one was
lingering until some other event closed the connection.
Adding an extra read where we expect to hit the end of file makes the HTTP
connection ready for reuse. This thus plugs a real socket leak.
To distinguish HTTP from SSH we look at self's class, just like it is done in
putlfile.