staking: validator definitions with conflicting consensus keys

[erwanor]

Is your feature request related to a problem? Please describe.
Similar to #3855, Zellic found that it is possible to craft a transaction with two actions that refer to distinct identity keys but conflicting consensus keys. This is a high-priority fix, because CometBFT will halt if presented with duplicate consensus identities.

[hdevalence]

We already have logic to prevent duplicate consensus keys. Did it not work here?

[erwanor]

@hdevalence This is another TOCTOU hazard, where it's possible to have two actions that use different identity keys (= different definitions) but colliding consensus keys.

[zbuc]

We check the consensus key isn't a duplicate in the verify_stateful method for the ValidatorDefinition action so this is a little surprising. I'll try to replicate

[hdevalence]

verify_stateful is precisely where the TOCTOU comes from — its parallelized

[hdevalence]

see eg the comments in the actionhandler impl in #3883

[hdevalence]

maybe we should rename the method later

[hdevalence]

We will do #3960 instead.