Add BlockState2

[wezrule]

Will need updating once state blocks v2 are in (nanocurrency/nano-node#2899). 8 more bytes needed, which are located right before the signature field:
Height (6 bytes)
Version (1 byte)
Flags (1 byte)

[rotilho]

Aren't signer and link interpretation part of the block? It would be great to have both of them so the node can validate the transaction in early stages

[wezrule]

Yes Flags contains those things (and whether the block is an upgrade)