Msgs and routes are disabled via an antehandler decorator. The decorator checks incoming all txs and rejects them if they contain a disallowed msg type.
Disallowed msg types are stored in a circuit breaker keeper.
The list of disallowed msg types is updated via a custom governance proposal and handler.
Design Alternatives:
- store list of disallowed msg types in params, then don't need custom gov proposal
- replace the app Router with a custom one to avoid using the antehandler - can't be done with current baseapp, but v0.38.x enables this. (https://github.com/cosmos/cosmos-sdk/issues/5455)
