Cell reload / SIGHUP handler (SYN-5886)

[vEpiphyte]

• Add Cell + CellAPI apis for listing and running reload functions
• Cell.addHttpsPort now adds a reload function to reload the SSL certificates when the SSL context is not provided
• Cell.addSignalHandlers now adds a SIGHUP handler to call all registered reload functions
• Add synapse.tools.reload to interact with a services reload functions

[codecov]

Codecov Report

Patch coverage: 98.30% and project coverage change: -0.10% ⚠️

Comparison is base (c31fa82) 97.32% compared to head (da6b442) 97.22%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3293      +/-   ##
==========================================
- Coverage   97.32%   97.22%   -0.10%     
==========================================
  Files         227      228       +1     
  Lines       45496    45613     +117     
==========================================
+ Hits        44277    44346      +69     
- Misses       1219     1267      +48     

Flag	Coverage Δ
linux	97.22% <98.30%> (+<0.01%)	⬆️
linux_replay	?

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Files Changed	Coverage Δ
synapse/lib/cell.py	98.05% <97.95%> (-0.01%)	⬇️
synapse/tools/reload.py	97.95% <97.95%> (ø)
synapse/tests/utils.py	94.64% <100.00%> (-1.71%)	⬇️

... and 8 files with indirect coverage changes

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[MichaelSquires]

Looks great! Two minor comments 😄

[MichaelSquires]

I left some suggestions but they're pedantic. Feel free to accept or not :)