When sending final block sync bundle, indicate if the last batch of blocks is 'final'

[Vagabond]

Currently the block sender does not indicate if the final batch of blocks sent is all the block the far end has. Adding a flag to the protocol and having the client check for that to inform further sync behaviour would be useful. Currently the node waits for a new block hash gossip to try to sync again, which may take several minutes.

[Vagabond]

Additionally, if the last block is not a final block, the node does not need to "eagerly gossip" it.