Initial open fd monitoring w/ cell write lock (SYN-10120)

[vEpiphyte]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 80.88235% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.66%. Comparing base (ff2b3a6) to head (277b553).

Files with missing lines	Patch %	Lines
synapse/lib/platforms/darwin.py	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4684      +/-   ##
==========================================
- Coverage   97.72%   97.66%   -0.06%     
==========================================
  Files         296      296              
  Lines       62114    62172      +58     
==========================================
+ Hits        60701    60721      +20     
- Misses       1413     1451      +38     

Flag	Coverage Δ
linux	97.63% <80.88%> (-0.03%)	⬇️
linux_replay	32.32% <60.29%> (-61.06%)	⬇️

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[MichaelSquires]

We should disable/not start the FD monitoring if the limit is already set to unlimited.

[vEpiphyte]

@MichaelSquires that would be safe if we assume that no one will ever change the soft limit of the process while it is running. If we do decide to bail in that case we should include in docs that monitoring isn't done if the soft limit is set to unlimited.