stop forking a crazy number of kid layers that blows up cluster tests with max file descriptors

[invisig0th]

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[codecov]

Codecov Report

Merging #1977 (9cfc083) into master (db03470) will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1977      +/-   ##
==========================================
- Coverage   96.75%   96.60%   -0.15%     
==========================================
  Files         146      146              
  Lines       27795    27795              
==========================================
- Hits        26893    26852      -41     
- Misses        902      943      +41     

Flag	Coverage Δ
linux	96.60% <ø> (-0.15%)	⬇️
linux_replay	?

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Impacted Files	Coverage Δ
synapse/tests/utils.py	93.59% <0.00%> (-2.18%)	⬇️
synapse/lib/hiveauth.py	95.96% <0.00%> (-0.77%)	⬇️
synapse/lib/view.py	95.37% <0.00%> (-0.73%)	⬇️
synapse/cortex.py	95.59% <0.00%> (-0.72%)	⬇️
synapse/lib/trigger.py	93.91% <0.00%> (-0.44%)	⬇️
synapse/lib/lmdbslab.py	99.39% <0.00%> (-0.21%)	⬇️
synapse/lib/layer.py	97.81% <0.00%> (-0.07%)	⬇️

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