Progress ETA is very wrong if upload is restarted. (lp:#1789153)

[Original report](https://bugs.launchpad.net/bugs/1789153) created by **Jeff Johnson (jeffjohnson0)**
```
When aborting an upload and the next invocation requires a 'RESTART', the
ETA is very wrong and never seems to correct it self.  In the log below, it
seems that the first 23% is done in 0 seconds, which skews the ETA.  Every
30 seconds or so it will add a minute.  This upload should take ~18 hours.
It would be nice if the RESTART didn't include the already completed
percentage.


RESTART: Volumes 99 to 100 failed to upload before termination.
         Restarting backup at volume 99.
Restarting after volume 98, file XXXXXXXXXX, block 9016
0.0KB 00:00:00 [0.0KB/s] [=========>                               ] 23%
ETA 0sec
0.0KB 00:00:10 [0.0KB/s] [=========>                               ] 23%
ETA < 45sec
8.7MB 00:00:20 [267.9KB/s] [=========>                               ] 23%
ETA 1min
18.5MB 00:00:30 [488.0KB/s] [=========>                               ] 23%
ETA 1min 30sec
28.0MB 00:00:40 [631.5KB/s] [=========>                               ] 23%
ETA 2min
36.4MB 00:00:50 [702.1KB/s] [=========>                               ] 23%
ETA 2min 30sec
46.0MB 00:01:00 [787.0KB/s] [=========>                               ] 23%
ETA 3min
55.7MB 00:01:10 [846.5KB/s] [=========>                               ] 23%
ETA 3min 30sec
65.6MB 00:01:20 [897.5KB/s] [=========>                               ] 23%
ETA 4min
75.1MB 00:01:30 [919.9KB/s] [=========>                               ] 23%
ETA 4min 30sec
84.8MB 00:01:40 [940.0KB/s] [=========>                               ] 23%
ETA 5min 30sec
94.2MB 00:01:50 [949.1KB/s] [=========>                               ] 23%
ETA 6min
104.0MB 00:02:00 [962.7KB/s] [=========>                               ]
23% ETA 6min
113.1MB 00:02:10 [954.2KB/s] [=========>                               ]
23% ETA 7min
123.3MB 00:02:20 [980.1KB/s] [=========>                               ]
23% ETA 7min
133.0MB 00:02:30 [983.1KB/s] [=========>                               ]
23% ETA 8min
142.6MB 00:02:40 [985.5KB/s] [=========>                               ]
23% ETA 8min
152.4MB 00:02:50 [989.2KB/s] [=========>                               ]
23% ETA 9min
161.9MB 00:03:00 [983.6KB/s] [=========>                               ]
23% ETA 9min
169.1MB 00:03:10 [911.3KB/s] [=========>                               ]
23% ETA 10min
178.8MB 00:03:20 [935.7KB/s] [=========>                               ]
23% ETA 11min
188.2MB 00:03:30 [943.7KB/s] [=========>                               ]
23% ETA 11min
197.2MB 00:03:40 [936.4KB/s] [=========>                               ]
23% ETA 12min
203.0MB 00:03:50 [832.0KB/s] [=========>                               ]
23% ETA 12min
212.8MB 00:04:00 [885.2KB/s] [=========>                               ]
23% ETA 13min
222.9MB 00:04:10 [928.2KB/s] [=========>                               ]
23% ETA 13min
232.0MB 00:04:20 [930.3KB/s] [=========>                               ]
23% ETA 14min
241.6MB 00:04:30 [944.2KB/s] [=========>                               ]
23% ETA 14min
251.5MB 00:04:40 [964.4KB/s] [=========>                               ]
23% ETA 15min
260.8MB 00:04:50 [961.1KB/s] [=========>                               ]
23% ETA 15min
271.3MB 00:05:00 [995.2KB/s] [=========>                               ]
23% ETA 16min
280.1MB 00:05:10 [966.6KB/s] [=========>                               ]
23% ETA 17min
289.5MB 00:05:20 [966.5KB/s] [=========>                               ]
23% ETA 17min
298.5MB 00:05:30 [951.0KB/s] [=========>                               ]
23% ETA 18min
307.8MB 00:05:40 [951.7KB/s] [=========>                               ]
23% ETA 18min
317.3MB 00:05:50 [959.1KB/s] [=========>                               ]
23% ETA 19min
326.8MB 00:06:00 [961.5KB/s] [=========>                               ]
23% ETA 19min
336.5MB 00:06:10 [972.5KB/s] [=========>                               ]
23% ETA 20min
346.5MB 00:06:20 [987.2KB/s] [=========>                               ]
23% ETA 20min
355.5MB 00:06:30 [965.8KB/s] [=========>                               ]
23% ETA 21min
365.0MB 00:06:40 [967.1KB/s] [=========>                               ]
23% ETA 22min
379.0MB 00:06:50 [1.1MB/s] [=========>                               ] 23%
ETA 22min
392.7MB 00:07:00 [1.2MB/s] [=========>                               ] 23%
ETA 23min
403.1MB 00:07:10 [1.1MB/s] [=========>                               ] 23%
ETA 23min
417.1MB 00:07:20 [1.2MB/s] [=========>                               ] 23%
ETA 24min
430.8MB 00:07:30 [1.3MB/s] [=========>                               ] 23%
ETA 24min
444.7MB 00:07:40 [1.3MB/s] [=========>                               ] 23%
ETA 25min
458.6MB 00:07:50 [1.3MB/s] [=========>                               ] 23%
ETA 25min



Ubuntu is version 16.04. Python is version 2.7.12. Duplicity is version
0.7.17. b2 backend is 1.3.4.
```

Story points: 6.0