Before this change, using -P occasionally deadlocked on the Transfer
mutex when Transfer.Done() was called with a non nil error and the
StatsInfo mutex since they mutually call each other.
This was fixed by making sure that the Transfer mutex is always
released before calling any StatsInfo methods.
This improves on: 6f87267b34Fixes#3505
Before this change, using -P occasionally deadlocked on the transfer
mutex and the stats mutex since they call each other via the progress
printing.
This is fixed by shortening the locking windows and converting the
mutex to a RW mutex.
This is done to make clear ownership over accounting object and prepare
for removing global stats object.
Stats elapsed time calculation has been altered to account for actual
transfer time instead of stats creation time.
