Initialize layer slabs before opening in readonly mode

[Cisphyx]

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.10 ⚠️

Comparison is base (1308584) 97.27% compared to head (6882b85) 97.18%.

❗ Current head 6882b85 differs from pull request most recent head 3f2617b. Consider uploading reports for the commit 3f2617b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3106      +/-   ##
==========================================
- Coverage   97.27%   97.18%   -0.10%     
==========================================
  Files         222      222              
  Lines       44057    44065       +8     
==========================================
- Hits        42857    42823      -34     
- Misses       1200     1242      +42     

Flag	Coverage Δ
linux	97.18% <100.00%> (+<0.01%)	⬆️
linux_replay	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
synapse/lib/layer.py	96.61% <100.00%> (-0.03%)	⬇️

... and 9 files with indirect coverage changes

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