client: Less noisy progress

[nirs]

Previously we updated the progress up to 10 times per second, and we
displayed fractional percent values (e.g. 12.57%). This is too noisy and
creates too many uninteresting updates. It also calls
time.monotonic_time() on every update to check if it is time to update
which is waste of resources for very little benefit.

Change the progress to show integer values (12%) and update the progress
only when the progress value changes. With this change we update the
progress up to 100 times during a transfer.

This PR implements part of #72.

[aesteve-rh]

Nice!

[rokkbert]

I think it's better like this and it looks correct to me, except for some typos and an outdated comment.
But the purpose of line.ljust(self.width, " ") is not clear to me. It's not changed in this PR though, so doesn't matter maybe.

[nirs]

Example output when downloading real image:

$ ./ovirt-img download-disk -c engine adb9b2c7-32a9-4e87-894c-710de7165086 \
    /data/fc/tmp/dl.qcow2 | tr '\r' '\n'
[ ---- ] 0 bytes, 0.00 seconds, 0 bytes/s                                      
[   0% ] 960.00 KiB, 0.06 seconds, 17.01 MiB/s                                 
[   2% ] 159.12 MiB, 0.06 seconds, 2.81 GiB/s                                  
[   6% ] 415.00 MiB, 0.06 seconds, 7.32 GiB/s                                  
[  10% ] 663.38 MiB, 0.06 seconds, 11.69 GiB/s                                 
[  14% ] 914.75 MiB, 0.06 seconds, 16.11 GiB/s                                 
[  15% ] 932.06 MiB, 0.06 seconds, 16.39 GiB/s                                 
[  30% ] 1.81 GiB, 0.06 seconds, 28.14 GiB/s                                   
[  31% ] 1.86 GiB, 0.21 seconds, 8.91 GiB/s                                    
[  44% ] 2.68 GiB, 0.25 seconds, 10.84 GiB/s                                   
[  45% ] 2.74 GiB, 0.42 seconds, 6.54 GiB/s                                    
[  46% ] 2.81 GiB, 0.59 seconds, 4.74 GiB/s                                    
[  64% ] 3.88 GiB, 0.61 seconds, 6.34 GiB/s                                    
[  65% ] 3.95 GiB, 0.68 seconds, 5.83 GiB/s                                    
[  66% ] 3.96 GiB, 0.70 seconds, 5.64 GiB/s                                    
[  67% ] 4.03 GiB, 0.72 seconds, 5.59 GiB/s                                    
[  68% ] 4.11 GiB, 0.77 seconds, 5.35 GiB/s                                    
[  83% ] 5.03 GiB, 0.85 seconds, 5.96 GiB/s                                    
[  84% ] 5.05 GiB, 0.87 seconds, 5.79 GiB/s                                    
[  86% ] 5.17 GiB, 0.88 seconds, 5.85 GiB/s                                    
[  87% ] 5.25 GiB, 1.20 seconds, 4.38 GiB/s                                    
[  88% ] 5.29 GiB, 1.20 seconds, 4.40 GiB/s                                    
[  89% ] 5.37 GiB, 1.47 seconds, 3.65 GiB/s                                    
[  91% ] 5.50 GiB, 1.64 seconds, 3.36 GiB/s                                    
[  92% ] 5.57 GiB, 1.73 seconds, 3.23 GiB/s                                    
[  93% ] 5.62 GiB, 1.73 seconds, 3.25 GiB/s                                    
[  94% ] 5.69 GiB, 1.83 seconds, 3.10 GiB/s                                    
[  95% ] 5.70 GiB, 1.99 seconds, 2.86 GiB/s                                    
[  96% ] 5.80 GiB, 2.25 seconds, 2.58 GiB/s                                    
[  97% ] 5.83 GiB, 2.35 seconds, 2.49 GiB/s                                    
[  99% ] 5.94 GiB, 2.39 seconds, 2.49 GiB/s                                    
[ 100% ] 6.00 GiB, 2.42 seconds, 2.48 GiB/s                                    
[ 100% ] 6.00 GiB, 2.45 seconds, 2.45 GiB/s