Change open_and_get_aux to open DB in readonly mode

[mappum]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.83%. Comparing base (97fca3d) to head (a914255).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/merk/mod.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #85      +/-   ##
===========================================
- Coverage    92.85%   92.83%   -0.02%     
===========================================
  Files           26       26              
  Lines         5611     5612       +1     
===========================================
  Hits          5210     5210              
- Misses         401      402       +1     

Files with missing lines	Coverage Δ
src/merk/mod.rs	83.93% <0.00%> (-0.14%)	⬇️