Account for state availability when fetching sync committees

[michaelsproul]

Short placeholder issue (will add more later):

Lighthouse currently loads the state from the start of the sync committee period when sync duties are requested. This can cause confusion when that state is not available, but a state from the same period is available. We could use the state_upper_limit to bound the slot of the state fetched, something like: max(sync_committee_start_slot, state_upper_limit) (if state_upper_limit != INT_MAX).

[Ayushdubey86]

can i look into this?

[michaelsproul]

@Ayushdubey86 sure!

[Ayushdubey86]

Hey @michaelsproul , can you check out my pr if possible!