Introduced user download_cps property

Will track the user's last succsesful file-download transfer rate
in characters (bytes) per second.

I'm not calculating or storing this rate yet, but will be soon. This
will make the file download ETAs more realistic and no longer
hard-coded to 3000 cps (which now defaults to 10000 cps, to keep up
with the times).

New field added to user.tab placed adjacent to the other File xfer stats,
which means the leech attempt counter (likely always 0) was moved to the end
of the user record.