curl_easy_perform: gradually increase the delay time

Instead of going 50,100,150 etc millisecond delay time when nothing has
been found to do or wait for, we now start lower and double each loop as
in 4,8,16,32 etc.

This lowers the minimum wait without sacrifizing the longer wait too
much with unnecessary CPU cycles burnt.

Bug: http://curl.haxx.se/mail/lib-2013-07/0103.html
Reported-by: Andreas Malzahn