Fix up client demo

[garious]

A few fixes:

• nonblocking tcp client
• purge broken pipes from subscriber list
• don't wait for default sig if no transactions were sent

[codecov-io]

Codecov Report

Merging #86 into master will increase coverage by 0.11%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   90.28%   90.39%   +0.11%     
==========================================
  Files          15       15              
  Lines        1163     1166       +3     
==========================================
+ Hits         1050     1054       +4     
+ Misses        113      112       -1

Impacted Files	Coverage Δ
src/accountant_stub.rs	61.76% <0%> (+0.59%)	⬆️
src/accountant_skel.rs	89.38% <100%> (+0.38%)	⬆️

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