This fixes several things wrong with the layout of the stats.
Transfers which haven't started are printed in the same format as
those which have so the stats with `--progress` don't show horrible
artifacts.
Checkers and transfers now get a ": checkers" and ": transfers" label
on the end of the stats line. Transfers will have the transfer stats
when the transfer has started instead of this.
There was a bug in the routine which shortened the file names (it
always produces strings 1 too long). This is now fixed with a test.
The formatting string was wrong with a fixed width of 45 - this is now
replaces with the value of `--stats-file-name-length`.
This also meant that there were unecessary leading spaces in the file
names. So the default `--stats-file-name-length` was raised to 45
from 40.
This removes the old system of part accounting and replaces it with a
system of popping off the accounting reader and wrapping up new ones
as necessary.
This makes it much easier to carry the context down the chain of
wrapped readers and get the limiting as near as possible to the
output. This makes the accounting more accurate and the bandwidth
limiting smoother.
Fixes#2029 and Fixes#1443
