Adding the `no_holes` and `no_holes_read_twice` flags

[giovannipizzi]

These flags are added to the two functions to write directly to packs add_streamed_objects_to_pack and add_objects_to_pack.

In this way, it is possible to have all three behaviors:

• we are efficient and we read the data only once, but we can occupy a lot of space
• we are efficient and we read only once, and we go back on disk if the object exists, but we risk to damage the disk if we write a lot of times on the same spot
• we accept a performance hit, but we never write to disk objects that already exist. This requires a read stream that allows seeking back to zero.

Fixes #85

[codecov[bot]]

Codecov Report

Merging #89 into develop will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           develop       #89   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines         1175      1199   +24     
=========================================
+ Hits          1175      1199   +24     

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

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