Add some immutable upload eliot logging

[exarkun]

Fixes: ticket:3023

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This change is 

[meejah]

The linter is sad about one import. Otherwise, 👍

[codecov-io]

Codecov Report

Merging #594 into master will decrease coverage by 0.83%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #594      +/-   ##
==========================================
- Coverage   86.17%   85.33%   -0.84%     
==========================================
  Files         159      159              
  Lines       28654    29586     +932     
  Branches     4076     4422     +346     
==========================================
+ Hits        24692    25248     +556     
- Misses       3264     3608     +344     
- Partials      698      730      +32

Impacted Files	Coverage Δ
src/allmydata/immutable/encode.py	95.95% <100%> (ø)	⬆️
src/allmydata/immutable/upload.py	95.63% <100%> (+0.1%)	⬆️
src/allmydata/windows/inotify.py	51.51% <0%> (-21.57%)	⬇️
src/allmydata/util/fake_inotify.py	96.92% <0%> (-0.91%)	⬇️
src/allmydata/frontends/sftpd.py	15.56% <0%> (+3.47%)	⬆️
src/allmydata/frontends/magic_folder.py	92.49% <0%> (+3.98%)	⬆️

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