Reduce head state copy for attester and aggregator

[terencechain]

🚀 Feature Request

Local memory for 4k validators slowly grow:

From profiling heap, one weakness is the amount of head state copy in RPC to service attester and aggregator:

This can be greatly improved by caching only one head state and use that across attester and aggregator

[terencechain]

Started to look into this. I think there's seriously gain we can achieve. Imagine all the highlighted flame goes away (2GB). Depends on how many validators the beacon node serves, This will reduce state copy from assigned_committees_count to 1 every slot.