Future light client tests will also incorporate execution payload data.
To avoid confusion, rename the current `root` check to `beacon_root`.
Doing this now, as #3066 already requires LC test runners to update.
Replaces `process_slot_for_light_client_store` which force updates the
`LightClientStore` automatically based on `finalized_header` age with
`try_light_client_store_force_update` which may be manually called based
on use case dependent heuristics if light client sync appears stuck.
Not all use cases share the same risk profile.
Etan Kissling
Hsiao-Wei Wang