EPIC: Support BN start from a saved state

[Nana-EC]

Problem

The block node must maintain correct state, as such a node connecting must be able to get state before processing the block stream.

Solution

Explore a way to start/re-start from saved state

Estimation consideration

1. Similar to Reconnect on CN
2. Catch up from another BN if necessary
3. Catch up from CN when possible.
4. Continue from CN once caught up if available.

Alternatives

No response