[Windows] batch multiple process info in one shot

[giampaolo]

This is a reminder ticket to continue the discussion started in #2366.
Recap: on Windows we use NtQuerySystemInformation(SystemProcessInformation) to collect multiple process info about all PIDs (Windows is the only platform which allows this). This data structure is used on a per-process basis, meaning it's discarded (or to put it another way, it's unnecessarily fetched for every Process instance).

psutil.process_iter() could be tweaked in a way that it passes the entire structure to the Process instances, and this way we would have a performance boost. In detail, read the considerations written in #2366 (comment).