Handle validators not in the state at startup

[rodrigo-o]

Validators that are not in the beacon state at the node initialization are ignored. This needs to be handled in the ValidatorSet probably at notify_head. We can use the store in this case to check if uninitialized validators are now in the state. Here are a couple of comments referring to this issue:

• refactor: validator state management refactor #1274 (comment)
• refactor: validator state management refactor #1274 (comment)
• refactor: validator state management refactor #1274 (comment)