multi: make Curl_expire() work with 0 ms timeouts

Previously, passing a timeout of zero to Curl_expire() was a magic code
for clearing all timeouts for the handle. That is now instead made with
the new Curl_expire_clear() function and thus a 0 timeout is fine to set
and will trigger a timeout ASAP.

This will help removing short delays, in particular notable when doing
HTTP/2.