curl_easy_perform: fix timeout handling

curl_multi_wait() was erroneously used from within
curl_easy_perform(). It could lead to it believing there was no socket
to wait for and then instead sleep for a while instead of monitoring the
socket and then miss acting on that activity as swiftly as it should
(causing an up to 1000 ms delay).

Reported-by: Antoni Villalonga
Fixes #3305
Closes #3306
Closes #3308