Upgrade checking handlers for Stoa

[linked0]

After shutting down nodes, there are cases where nodes crash
because the handlers for Stoa are nullified. So this is a way
to keep from crashing by using the handler structure itself, not
the index.

Fixes #2942

[codecov]

Codecov Report

Merging #3165 (a33b4a9) into v0.x.x (c60c869) will increase coverage by 0.21%.
The diff coverage is 20.83%.

@@            Coverage Diff             @@
##           v0.x.x    #3165      +/-   ##
==========================================
+ Coverage   88.15%   88.37%   +0.21%     
==========================================
  Files         165      165              
  Lines       17067    17079      +12     
==========================================
+ Hits        15045    15093      +48     
+ Misses       2022     1986      -36     

Flag	Coverage Δ
integration	39.37% <0.00%> (+2.57%)	⬆️
unittests	87.74% <20.83%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
source/agora/node/FullNode.d	73.41% <20.83%> (+0.96%)	⬆️
source/agora/consensus/BlockStorage.d	75.61% <0.00%> (-1.77%)	⬇️
source/agora/consensus/protocol/Nominator.d	90.94% <0.00%> (-0.54%)	⬇️
source/agora/script/Engine.d	97.34% <0.00%> (-0.15%)	⬇️
source/agora/test/Base.d	80.70% <0.00%> (+0.22%)	⬆️
source/agora/network/Manager.d	77.21% <0.00%> (+0.31%)	⬆️
source/agora/consensus/state/Ledger.d	89.50% <0.00%> (+0.65%)	⬆️
source/agora/node/Validator.d	92.46% <0.00%> (+1.00%)	⬆️
source/agora/node/Config.d	68.61% <0.00%> (+3.64%)	⬆️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c60c869...a33b4a9. Read the comment docs.

[linked0]

Ready for review. The spurious error on CI happens tho.

[Geod24]

This is not fixing the issue IMO, for the same reason that previous changes didn't fix the issue.
After Ctrl+C is called and the shutdown is initiated, nothing should be called, because we cannot sanely shut down in the signal handler if things are going to fire randomly afterwards. However, we're seeing via this segv that things DO get called. Additionally, #3158 seems to suggest that this trigger in prod, too.