Transition from 'Initial Sync' to 'Chain Follow'

[amerameen]

Acceptance Criteria

• Once 'initial sync' is done, switch the ChainSync state to ChainFollow, which will allow pubsub messages to be processed from the network event channel

Questions

• How do we know when intial sync is done? May be worthwhile looking into how Lotus does it.
• Do we need to do another 'Initial Sync' after the first one is done (since it takes a while) before switching 'ChainFollow'?
• The whole time we're doing 'Initial Sync' we're getting gossipsub blocks, what is happening to these? What do we do with them once we switch to 'ChainFollow'?
• How to test this properly? May be difficult
• Signalling to the ChainSyncer that 'Initial Sync' is done is not trivial.