Optimize GetDuties VC action

[rkapka]

What type of PR is this?

Other

What does this PR do? Why is it needed?

This PR reduces the time to fetch duties from ~8-9s to ~2.5s by utilizing wait groups and making concurrent requests. Concurrency is spread between two layers:

1. Fetch duties for current and next epoch concurrently
2. Within each epoch, fetch attester duties, proposer duties, sync committee duties and committees concurrently

Another piece of this PR is a getValidatorsForDuties function that is used instead of multipleValidatorStatus, This is because the latter does many things that we don't need for duties, including a call to GetValidatorCount which currently takes around 250ms on Holesky.

[nalepae]

Question:

Why using an errgroup here for only one goroutine?
(In general errgroups are used when several goroutines run in parallel, and we want manage the case one of them returns an error.)

A pattern without errgroup could be the following:

errCh := make(chan error)  // Create a channel for errors
 go faultyFunction(errCh) // Start the faulty function in a new goroutine

 // Do other stuffs

 err := <-errCh  // Wait for an error
 fmt.Println("Received an error:", err)

With faultyFunction sending the error at the end in errCh.

However, I admit errgroup totally works here.

[nalepae]

Just as a safety measure, I would define:

• attesterDutiesMapping,
• syncDutiesMapping,
• proposerDutySlots and
• committeeMapping

at the same place in the code, and add a commentary saying that the main routine cannot use these variables before wg.Wait().

(Such a fact sounds obvious now, but in a few months/years an other developer may want to use these variables before the wg.Wait() and some bad things may happen in this case.)

[nalepae]

Suggested change

	var attesterSlot primitives.Slot
	var committeeIndex primitives.CommitteeIndex
	var committeeValidatorIndices []primitives.ValidatorIndex
	var (
	attesterSlot primitives.Slot
	committeeIndex primitives.CommitteeIndex
	committeeValidatorIndices []primitives.ValidatorIndex
	)