Reject block.nVersion<=1 blocks if network has upgraded to version=2

If 950 of the last 1,000 blocks are nVersion=2, reject nVersion=1
(or zero, but no bitcoin release has created block.nVersion=0) blocks

• 75 of last 100 on testnet3.

This rule is being put in place now so that we don't have to go
through another "express support" process to get what we really
want, which is for every single new block to include the block height
in the coinbase.