WIP: Cell log access APIs ( prototype for discussion )

[invisig0th]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 86.79245% with 7 lines in your changes missing coverage. Please review.

Project coverage is 96.39%. Comparing base (42eccab) to head (59bb875).
Report is 120 commits behind head on master.

Files with missing lines	Patch %	Lines
synapse/lib/cell.py	86.79%	7 Missing ⚠️

❌ Your patch status has failed because the patch coverage (86.79%) is below the target coverage (96.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4061      +/-   ##
==========================================
- Coverage   96.48%   96.39%   -0.09%     
==========================================
  Files         256      256              
  Lines       59645    59698      +53     
==========================================
  Hits        57548    57548              
- Misses       2097     2150      +53     

Flag	Coverage Δ
linux	96.39% <86.79%> (-0.02%)	⬇️
linux_replay	?

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[vEpiphyte]

n.b. I want to steal some of this for #3922

[vEpiphyte]

Close for #4133