update the section on timeouts

  information about what socket to wait for what action on, and the callback

  only gets called if the status of that socket has changed.

  In the API draft from before, we have a timeout argument on a per socket

  basis and we also allowed curl_multi_socket_action() to pass in an 'easy

  handle' instead of socket to allow libcurl to shortcut a lookup and work on

  the affected easy handle right away. Both these turned out to be bad ideas.

  The timeout argument was removed from the socket callback since after much

  thinking I came to the conclusion that we really don't want to handle

  timeouts on a per socket basis. We need it on a per transfer (easy handle)

  basis and thus we can't provide it in the callbacks in a nice way. Instead,

  we have to offer a curl_multi_timeout() that returns the largest amount of

  time we should wait before we call the "timeout action" of libcurl, to

  trigger the proper internal timeout action on the affected transfer. To get

  this to work, I added a struct to each easy handle in which we store an

  "expire time" (if any). The structs are then "splay sorted" so that we can

  add and remove times from the linked list and yet somewhat swiftly figure

  out 1 - how long time there is until the next timer expires and 2 - which

  timer (handle) should we take care of now. Of course, the upside of all this

  is that we get a curl_multi_timeout() that should also work with old-style

  applications that use curl_multi_perform().

  We also added a timer callback that makes libcurl call the application when

  the timeout value changes, and you set that with curl_multi_setopt().

  the timeout value changes, and you set that with curl_multi_setopt() and the

  CURLMOPT_TIMERFUNCTION option. To get this to work, Internally, there's an

  added a struct to each easy handle in which we store an "expire time" (if

  any). The structs are then "splay sorted" so that we can add and remove

  times from the linked list and yet somewhat swiftly figure out both how long

  time there is until the next nearest timer expires and which timer (handle)

  we should take care of now. Of course, the upside of all this is that we get

  a curl_multi_timeout() that should also work with old-style applications

  that use curl_multi_perform().

  We created an internal "socket to easy handles" hash table that given

  a socket (file descriptor) return the easy handle that waits for action on