Fix concurrency problems on Windows

[giovannipizzi]

Essentially, one of the main issues is that os.rename is atomic on Windows, but while renaming, the file is locked. So there is the possibility that any open statement will fail.

We now create a new concept of a duplicates folder, where we store e.g. a new loose object (one copy per failed attempt, with name <hashkey>.<random-uuid>) when we fail to create the loose object (typically during concurrent access) and we cannot check if the current existing object is correct.

Tests with mocking are added to have full coverage (if run on Windows as well).

After this is merged, #72 still needs to be addressed to clean up these copies. Also this folder should be considered in #13 when validating containers.

Fixes #37

[codecov[bot]]

Codecov Report

Merging #77 into develop will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           develop       #77   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          987      1010   +23     
=========================================
+ Hits           987      1010   +23     

Impacted Files	Coverage Δ
disk_objectstore/exceptions.py	100.00% <ø> (ø)
disk_objectstore/container.py	100.00% <100.00%> (ø)
disk_objectstore/utils.py	100.00% <100.00%> (ø)

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