core/deadline: ignore deadlined duties

[xenowits]

• Modify Deadliner to ignore deadlined duties.
• Modify Add(duty) to return true if a duty is added successfully.
• Fix deadline tests.

category: refactor
ticket: #814

[codecov]

Codecov Report

Merging #819 (25bb192) into main (fa8712e) will increase coverage by 0.14%.
The diff coverage is 85.07%.

@@            Coverage Diff             @@
##             main     #819      +/-   ##
==========================================
+ Coverage   54.58%   54.73%   +0.14%     
==========================================
  Files         111      111              
  Lines       11347    11384      +37     
==========================================
+ Hits         6194     6231      +37     
+ Misses       4251     4249       -2     
- Partials      902      904       +2     

Impacted Files	Coverage Δ
core/tracker/tracker.go	74.37% <0.00%> (-1.73%)	⬇️
core/deadline.go	64.86% <89.06%> (-3.14%)	⬇️
core/qbft/qbft.go	81.11% <0.00%> (-0.86%)	⬇️
dkg/transport.go	66.66% <0.00%> (+12.96%)	⬆️
core/types.go	37.61% <0.00%> (+13.76%)	⬆️

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