Replace deprecated pycurl IOCTLFUNCTION callback with SEEKFUNCTION

[takluyver]

Tests were failing with pycurl 7.46 because of a deprecationwarning with IOCTLFUNCTION. The suggested replacement is SEEKFUNCTION.

[takluyver]

In the second commit, the recommendation in curl's docs is to replace socket_all() with socket_action(), but I'm guessing a bit as to how exactly that should work; I'm not very familiar with (py)curl.

[bdarnell]

First commit looks good, thanks!

For the second commit, the use of socket_all instead of socket_timeout was originally deliberate. In 2010 there were bugs in which libcurl's internal timeout bookkeeping would lose track of a socket, and so we put in a periodic call to socket_all to force it to check everything again. Calling socket_action(SOCKET_TIMEOUT) is already done in handle_timeout, so doing it again in handle_force_timeout is redundant (assuming this logic in handle_timeout to handle differences between clock implementations is doing its job).

I think at this point I'm comfortable deleting everything related to force_timeout here. In 2010 we were kind of on the cutting edge of async libcurl usage and would run into a lot of bugs. But since then, aside from the general increasing maturity of libcurl, it's also been through a major refactoring (in 2013) so that the synchronous interfaces are implemented in terms of the async ones, so that class of bugs is less likely to go unnoticed.

[takluyver]

Makes sense, thanks. I've replaced that commit with one removing the force_timeout mechanism.