Fix CommitteeAssignments to not return every validator

[terencechain]

Currently, CommitteeAssignments returns every active validator in the beacon state. With 1 million validators, this easily exceeds 32 MB in the heap. This PR addresses the issue by refactoring CommitteeAssignment into three parts:

• verifyAssignmentEpoch: Verifies the requested epoch is correct with respect to the input beacon state.
• ProposerAssignments: Takes state and epoch as inputs, uses verifyAssignmentEpoch, and returns map[primitives.ValidatorIndex][]primitives.Slot. All proposers of the epoch are returned, with only one proposer per slot.
• CommitteeAssignments: Takes state, epoch, and requested_indices as inputs, uses verifyAssignmentEpoch, and returns map[primitives.ValidatorIndex]*CommitteeAssignment. The output is bounded by requested_indices.

Note to the reviewer: In ProposerAssignments, at the end of the function, I reset the beacon state to its original slot to prevent the function from mutating the slot.

// Reset state back to its original slot.
if err := state.SetSlot(originalStateSlot); err != nil {
    return nil, err
}

The previous behavior was incorrect and caused many inconsistent test false positives. The previous behavior was:

currentProposalEpoch := epoch < nextEpoch
if !currentProposalEpoch {
    if err := state.SetSlot(state.Slot() - params.BeaconConfig().SlotsPerEpoch); err != nil {
        return nil, nil, err

I'll document the false test behavior and the bug in the code diff.

[terencechain]

This was wrong, but luckily it was not surfaced because the old CommitteeAssignments would mutate the slot. We should use the request epoch instead of the state slot for the query, and only update the next sync committee if the next epoch crosses the boundary as specified below.

[terencechain]

This was another test bug because the old CommitteeAssignments would mutate state slot. We should set the state slot here similar to what we did in the first test case

[potuz]

The function verifyAssignmentEpoch allows to check for a state that has a slot from a past epoch. However, we never perform an epoch transition on the state when checking and simply use SetSlot() which does not update balances. This could potentially give the wrong indices for both proposers and attesters when the state passed is from an epoch before the requested epoch.

[potuz]

This reads simpler as

Suggested change

	for i := primitives.Slot(0); i < params.BeaconConfig().SlotsPerEpoch; i++ {
	for slot := primitives.Slot(startSlot); slot < startSlot + params.BeaconConfig().SlotsPerEpoch; slot++ {

[potuz]

This allocates a very large slice for no reason other than requesting its length in the next line 240. We only need to get the number of committees for the slot. No allocations should be performed here.