Fix: duplicate events

[tippenein]

fix: #2810

This simply uses a HashSet for event_observers which means it might not be exactly what the issue asked for but I'm putting it here for discussion. It wouldn't be particularly hard to WARN in the case of inserting an event observer which is already specified, but maybe it's sufficient to just disallow it?

(didn't add any tests because it ends up just testing HashSet's insert..)

[obycode]

This change will just silently ignore a duplicate, but the issue requests that it errors and exits. This should be an easy addition. Can you add that @tippenein?

[CharlieC3]

@obycode Re-thinking my original issue request, this may actually be a better solution since it still guards against user error while preserving flexibility to run the binary.
Unless you think otherwise, I'd be happy with this as-is!

[obycode]

@obycode Re-thinking my original issue request, this may actually be a better solution since it still guards against user error while preserving flexibility to run the binary. Unless you think otherwise, I'd be happy with this as-is!

Works for me!

[codecov]

Codecov Report

Attention: 84 lines in your changes are missing coverage. Please review.

Comparison is base (b0f637f) 0.04% compared to head (fc61021) 0.04%.
Report is 2 commits behind head on develop.

Files	Patch %	Lines
testnet/stacks-node/src/tests/neon_integrations.rs	0.00%	40 Missing ⚠️
testnet/stacks-node/src/config.rs	0.00%	13 Missing ⚠️
testnet/stacks-node/src/tests/epoch_21.rs	0.00%	7 Missing ⚠️
testnet/stacks-node/src/tests/epoch_205.rs	0.00%	5 Missing ⚠️
stackslib/src/chainstate/stacks/db/transactions.rs	0.00%	2 Missing ⚠️
stackslib/src/net/api/tests/mod.rs	0.00%	2 Missing ⚠️
stackslib/src/util_lib/strings.rs	0.00%	2 Missing ⚠️
testnet/stacks-node/src/tests/epoch_22.rs	0.00%	2 Missing ⚠️
testnet/stacks-node/src/tests/epoch_24.rs	0.00%	2 Missing ⚠️
testnet/stacks-node/src/tests/stackerdb.rs	0.00%	2 Missing ⚠️
... and 7 more

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4076   +/-   ##
========================================
  Coverage     0.04%    0.04%           
========================================
  Files          401      401           
  Lines       286816   286818    +2     
========================================
  Hits           136      136           
- Misses      286680   286682    +2     

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