Reduce CurlMaxPipelinedRequestsPerService to 8.

Larger values, certainly the previous value of 32, caused
delays/stalls. Apparently either CDN or the lab punish
one still for making too many parallel requests *even* though
the server has a header with Keep-Alive: timeout=10 max=400 !

This lower value did not only give me a higher throughput,
it also seemed to solve the 100% stalls with all libcurl-bug
disasters following that.