543.connected enough

[meejah]

See also #543

This re-works how the "connected-enough" check works: instead of once, at startup it is monitored continuously. This is reflected in the "status" endpoint as well as affecting the behavior of our tahoe client (namely: it refuses to do mutable things if we're in a "degraded" state where less than a "happy" number of servers are connected).

[codecov]

Codecov Report

Merging #634 (ad57d20) into main (a3d2986) will increase coverage by 0.30%.
The diff coverage is 97.87%.

@@            Coverage Diff             @@
##             main     #634      +/-   ##
==========================================
+ Coverage   92.09%   92.39%   +0.30%     
==========================================
  Files          38       38              
  Lines        4112     4171      +59     
  Branches      545      551       +6     
==========================================
+ Hits         3787     3854      +67     
+ Misses        221      215       -6     
+ Partials      104      102       -2     

Flag	Coverage Δ
integration	75.04% <80.85%> (+0.16%)	⬆️

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Impacted Files	Coverage Δ
src/magic_folder/tahoe_client.py	93.83% <87.50%> (-0.83%)	⬇️
src/magic_folder/magic_file.py	96.46% <100.00%> (+0.45%)	⬆️
src/magic_folder/participants.py	91.75% <100.00%> (ø)
src/magic_folder/service.py	99.42% <100.00%> (+4.71%)	⬆️
src/magic_folder/status.py	94.93% <100.00%> (+1.18%)	⬆️
src/magic_folder/web.py	97.84% <100.00%> (+0.06%)	⬆️

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