Reduce number of DBS concurrent requests in MSTransferor pycurl

[amaltaro]

Impact of the new feature
MSTransferor

Is your feature request related to a problem? Please describe.
DBS is tightening up the throttling limits in DBSReader Global (decreasing it from 10 to 5 requests per backend/DN now):
dmwm/deployment#919

and we better decrease the number of concurrent requests made against DBS as well, to avoid getting the service in an endless failure mode created by the concurrency/throttling in place.

Describe the solution you'd like
Ideally, the number of concurrent requests should be made as a configuration parameter, which would propagate all the way down to the method actually creating the pycurl requests.

Extra: we should print the amount of requests we have to make and the number of concurrent requests getting created.

Describe alternatives you've considered
If too much work to make it configurable, I think it's okay to have those limits hardwired in the source code.

Additional context
none