Add release assert if btree cache is full

[sanebay]

Will add flip frequency to 30% of io delay.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.67% ⚠️

Comparison is base (85b234f) 32.77% compared to head (5a568ba) 32.11%.

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Additional details and impacted files

@@               Coverage Diff               @@
##           stable/v3.x     #168      +/-   ##
===============================================
- Coverage        32.77%   32.11%   -0.67%     
===============================================
  Files              133      125       -8     
  Lines            17161    16477     -684     
  Branches          2053     1976      -77     
===============================================
- Hits              5624     5291     -333     
+ Misses           10965    10630     -335     
+ Partials           572      556      -16     

Files Changed	Coverage Δ
src/engine/blkstore/blkstore.hpp	27.71% <0.00%> (ø)

... and 13 files with indirect coverage changes

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