Use protocol versions during sync

[ZenGround0]

Once we have the block height to protocol version mapping in the spec (#197) the syncing protocol should be upgraded so that syncing blocks of a given height is done with respect to the protocol version that is in scope for that block's height.

[anorth]

I'm not sure we need anything in the spec for this.

[mishmosh]

Closing for now, feel free to reopen if you feel this is still required.