Use SelectChain in Environment::report_equivocation

[andresilva]

See #9154.
In order to do this both prevote_equivocation and precommit_equivocation need to become async functions. That's an involved change that will need to happen in https://github.com/paritytech/finality-grandpa.

[andresilva]

The current approach of using client.info().best_hash should not lead to correctness issues. We use this for two things:

• Figuring out when the next authority set change happens. For this to return the correct result we only need to make sure that the given best block is a descendent of the latest finalized block. This is guaranteed by client.info() so using SelectChain would not change anything.
• Submitting the equivocation report. For this to work correctly we only need to make sure that the given block hash is recent enough that state is available.