Clean up Brokers before firing Tub.stopService Deferred

[exarkun]

Fixes https://foolscap.lothar.com/trac/ticket/274

See also https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2896

[codecov-io]

Codecov Report

Merging #38 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   84.14%   84.18%   +0.04%     
==========================================
  Files          60       60              
  Lines        8912     8924      +12     
  Branches     1372     1374       +2     
==========================================
+ Hits         7499     7513      +14     
+ Misses       1000      999       -1     
+ Partials      413      412       -1

Impacted Files	Coverage Δ
src/foolscap/pb.py	88.78% <100%> (+0.06%)	⬆️
src/foolscap/broker.py	84.91% <100%> (+0.51%)	⬆️
src/foolscap/referenceable.py	89.34% <0%> (+0.22%)	⬆️

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[warner]

looks good to me