multi: make curl_multi_info_read perform O(1)

multi: make curl_multi_info_read perform O(1)

Instead of looping over all attached easy handles, this now keeps a list
of messages in the multi handle. It allows curl_multi_info_read() to
perform O(1) no matter how many easy handles that are handled. This is
of importance since this function may be polled very frequently by apps
using the multi interface.