Magicfolder backup files

[meejah]

(Note: this is on top of the "multi-magic folder" branch, so ignore that commit -- and should be re-based once that's merged).

This fixes 2880 along with an integration test. You can trigger the bug the first time (and only this time) you re-start a client after a file has been added (and uploaded) in a magic-folder; then, an unnecessary .backup file will appear.

[meejah]

I still have no idea what the windows builder is sad about ...

[codecov-io]

Codecov Report

Merging #448 into master will decrease coverage by 0.04%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
- Coverage    90.1%   90.06%   -0.05%     
==========================================
  Files         144      144              
  Lines       27294    27301       +7     
  Branches     3926     3929       +3     
==========================================
- Hits        24594    24589       -5     
- Misses       1959     1967       +8     
- Partials      741      745       +4

Impacted Files	Coverage Δ
src/allmydata/util/deferredutil.py	70.73% <0%> (ø)	⬆️
src/allmydata/frontends/magic_folder.py	85.4% <100%> (+0.13%)	⬆️
src/allmydata/immutable/downloader/share.py	92.82% <0%> (-2.47%)	⬇️
src/allmydata/mutable/servermap.py	93.38% <0%> (-0.81%)	⬇️
src/allmydata/scripts/magic_folder_cli.py	69.64% <0%> (+0.25%)	⬆️
src/allmydata/immutable/upload.py	95.53% <0%> (+0.27%)	⬆️

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[meejah]

I don't know how to make codecov un-sad about the missing coverage; that commit did get built by travis -- perhaps that's when codecov was down and it didn't get the memo?

[meejah]

Should be good to merge

[warner]

LGTM