_Always_ (almost) use the tipset's parent state, instead of computing.
Exceptions:
* MinerGetBaseInfo. Fixing this would break things so we need to be
careful (although we could bump the API version, fix it, then fix the call
sites).
* StateReplay. This is replaying a message on top of the given tipset.
* GasEstimateGasLimit. This executes the message on-top-of the tipset's
computed state (unlike call which executes it on the tipset's parent state).
* Having this method and Call apply the message at different heights is really
weird.
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