multi: improved pending transfers handling => improved performance

When a transfer is requested to get done and it is put in the pending
queue when limited by number of connections, total or per-host, libcurl
would previously very aggressively retry *ALL* pending transfers to get
them transferring. That was very time consuming.

By reducing the aggressiveness in how pending are being retried, we
waste MUCH less time on putting transfers back into pending again.

Some test cases got a factor 30(!) speed improvement with this change.

Reported-by: Cyril B
Fixes #2369
Closes #2383