Simplify validator startup checks to wait for beacon node synced

[0xKiwi]

Currently the validator has multiple processes it runs before performing validator duties, these include: WaitForChainStart, WaitForSync and WaitForActivation.

WaitForSync and WaitForActivation both currently ping once for information, and after that they run every 6 seconds to check for the information the validator client needs. If the beacon node is not ready to return the information asked for at the time of request, the validator receives another response every 6 seconds after.

One issue discovered and explained to @rauljordan is that the first request of WaitForSync and WaitForActivation both fail initially after chain start, and fallback to the 6 second delay.

The validator should not use WaitForChainStart and WaitForSync for notification on when it should act if the beacon chain node is not ready yet by the time, we should instead use StateFeed.Initialized or a new StateFeed.Synced for tracking when the beacon node is ready to communicate with the validator client.