agreement: fix mainLoop vs Shutdown race

[algorandskiy]

Summary

Based on this test failure there one test started executing before the previous finished: mainLoop goroutine stayed alive after calling Shutdown.
This means the Shutdown, mainLoop and demuxLoop termination logic based on done channel is racy.

Fixed by replacing the done channel with explicit wait on a wait group.

Test Plan

Existing tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.96%. Comparing base (e25465b) to head (4ffdb73).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6349   +/-   ##
=======================================
  Coverage   51.96%   51.96%           
=======================================
  Files         653      653           
  Lines       87460    87461    +1     
=======================================
+ Hits        45445    45452    +7     
+ Misses      39141    39138    -3     
+ Partials     2874     2871    -3     

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