curl_multi regression since 7.29.0

[I got this reported privately and the reporter wants to keep its employer's name out of this. I'm filing it here so that I won't forget about this issue.]

https://github.com/bagder/curl/commit/cb3e6dfa3511e75334f70f53370ce3f1d711c904

The above commit introduced a pretty big performance regression when
dealing with large number of requests going through an HTTP proxy + ssl.
I¹ve measured around a 4.5x increase in time spent in the attached
application vs reverting this change. We noticed the problem when we
upgraded from 7.25 to 7.36 recently and it had a pretty negative impact on
some application logic we have. With the test application you will need
to change the proxy to your own local one of course.

Without ssl the attached application is very fast for me.. see the
following timings all based off of 7.37.0

time with https:
real 1m24.342s
user 0m12.911s
sys 0m2.362s

time with http:
real 0m1.480s
user 0m0.289s
sys 0m1.019s

time with https and cb3e6dfa3511e75334f70f53370ce3f1d711c904 reverted:
real 0m15.273s
user 0m12.872s
sys 0m2.248s

time with http and cb3e6dfa3511e75334f70f53370ce3f1d711c904 reverted:
real 0m1.441s
user 0m0.285s
sys 0m0.995s

(I'll attach the test application later after editing out sensitive names)