Fix missing Gossip subnet subscription on startup

[Nashatyrev]

PR Description

GossipForkManager has two different sources of isHeadOptimisticflag:

• RecentChainData.isChainHeadOptimistic() when starting up
• onOptimisticHeadChanged() to update the flag

There are possible cases (e.g. Nashatyrev#166) when the value differs in those two sources. Thus subnet subscriptions (which are to be activated when isHeadOptimistic becomes false) may not be activated.

This PR initializes the flag with RecentChainData.isChainHeadOptimistic() value and then relies on onOptimisticHeadChanged() notifications only

Fixed Issue(s)

Fix Nashatyrev#166

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[tbenr]

LGTM