Incrementally building clique blocks doesn't work

[gsalgado]

Trying to incrementally build clique blocks fails (the state root will be wrong on the final block) because the initial header is not signed so all tx fees are incorrectly assigned to the coinbase address.

[gsalgado]

We should also probably get rid of

py-evm/eth/consensus/clique/clique.py

Line 102 in 3b79b35

return header.coinbase

as it lets one incrementally build a block that in the end won't be valid