throttle: optional non perf counter mode

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: stream get_obj operation

Fixes: #2941
Instead of iterating through the parts one by one when reading
an object, we can now send multiple requests in parallel. Two new
configurables added to control the max request size, and the total
size of pending requests.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>