Final MiB/s transfer speed and file/s transfer rate inaccurate if backup is a continuation

[davedavemckay]

Line 393 in lsst-backup.py:

print(f'Total: {len(log_df)} files; {(final_size):.2f} MiB; {(final_size/final_time_seconds):.2f} MiB/s including setup time; {(final_time_seconds/len(log_df)):.2f} s/file including setup time')

reports:

Total: 488764 files; 56396520.10 MiB; 720.70 MiB/s including setup time; 0.16 s/file including setup time

when the true average speed is ~98 MiB/s and true file transfer rate is 1.2 s/file.
This is due to the upload being conducted over multiple runs and this stat being generated assuming "number_of_files / final_runtime".