Fix VM.GetBlockIDAtHeight

[StephenButtolph]

Why this should be merged

The specification for GetBlockIDAtHeight is:

	// GetBlockIDAtHeight returns:
	// - The ID of the block that was accepted with [height].
	// - database.ErrNotFound if the [height] index is unknown.
	//
	// Note: A returned value of [database.ErrNotFound] typically means that the
	//       underlying VM was state synced and does not have access to the
	//       blockID at [height].
	GetBlockIDAtHeight(ctx context.Context, height uint64) (ids.ID, error)

It should return database.ErrNotFound if the height index is unknown. Currently this function returns potentially arbitrary blkIDs when queried for heights greater than the last accepted height.

How this works

1. Immediately returns database.ErrNotFound for heights larger than the last accepted height.
2. Removes the reading of the block from disk, as it is not needed to serve this request.

How this was tested

• Added additional testing to the canonical height index.

[ceyonur]

lgtm.
e2e test should pass after this one gets merged #596