header: On connect hello msg with current known head to eagerly push it

[renaynay]

• Optimization to get heads as early as possible without requested msgs
• Allows analyzing rcvd heads and choosing the best as the sync target
• Helps with automatic subjective initialization(if the local head is expired) by choosing the most trustworthy head i.e. came from trusted peers or the majority of peers with high scores(served headers before) shared the same head. High scores here may help from an eclipse attack.