Fix exit verification at fork boundaries

[paulhauner]

Description

In #4183 we made a patch to fix high CPU usage for exit verification across skip slots.

This patch doesn't cover the case where we receive an exit for epoch E + 1 when the head state is in epoch E and those two epochs are across a fork boundary.

[michaelsproul]

I think the signature check is OK actually, because it uses the exit's internal epoch:

lighthouse/consensus/state_processing/src/per_block_processing/signature_sets.rs

Lines 390 to 395 in a53830f

	let domain = spec.get_domain(
	exit.epoch,
	Domain::VoluntaryExit,
	&state.fork(),
	state.genesis_validators_root(),
	);

But the other checks relative to current epoch are liable to fail, e.g.

lighthouse/consensus/state_processing/src/per_block_processing/verify_exit.rs

Lines 46 to 53 in a53830f

	// Exits must specify an epoch when they become valid; they are not valid before then.
	verify!(
	state.current_epoch() >= exit.epoch,
	ExitInvalid::FutureEpoch {
	state: state.current_epoch(),
	exit: exit.epoch
	}
	);

Plumbing an optional wall-clock epoch into verify_exit could be one way to verify future exits from the current head state

[paulhauner]

I've started working on a fix in #4183, so we might not need this issue after all.

[michaelsproul]

Closed by #4183