Nexusroot fini improvements (SYN-9990)

[vEpiphyte]

• During nexus teardown, await the apply task
• After nexus has been fini'd, prevent additional applytasks from executing

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.71%. Comparing base (5a56eef) to head (6bf0fe5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4640      +/-   ##
==========================================
- Coverage   97.72%   97.71%   -0.01%     
==========================================
  Files         295      295              
  Lines       62047    62057      +10     
==========================================
+ Hits        60634    60642       +8     
- Misses       1413     1415       +2     

Flag	Coverage Δ
linux	97.65% <100.00%> (+<0.01%)	⬆️
linux_replay	93.37% <100.00%> (-0.01%)	⬇️

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