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.
A deadlock could occur since we have now put a mutex on GetBytes from
StatsInfo.String (s.mu) - progress (acc.statmu) and read (acc.statmu)
- GetBytes (s.mu).
Fix this by giving stringSet its own locking and excluding the call
which caused the deadlock from the mutex in StatsInfo.String.
