Currently each multipart upload allocated his own buffers, which after
file upload was garbaged. Next files couldn't leverage already allocated
memory which resulted in inefficent memory management. This change
introduces backend memory pool keeping memory chunks which can be
used during object operations.
Fixes#3967
Statistics of ransfers which were interrupted are not cleared before
retry iteration. These transfers completed with over 100 percentage.
This change clears transfer accounting before next retry iteration is
done in order to keep numbers in track.
Fixes#3861
rclone library users might be intrested in changing default value to
other, or even disabling it. With current version it's impossible which
leads to races when number of uploaded objects exceeds default limit.
Fixes#3732
Maciej Zimnoch