Support PoA chains

[vpulim]

In Blockchain.prototype._putBlockOrHeader, it is assumed that the block is from a PoW chain:

    self.ethash.verifyPOW(block, function (valid) {
      next(valid ? null : new Error('invalid POW'))
    })

We need to properly handle PoA blocks. For now, we can simple skip the verifyPow call when block._common.consensus(hardfork) !== 'pow'

Also, depends on https://github.com/ethereumjs/ethereumjs-block/issues/45

[holgerd77]

@jochem-brouwer following your linked issue from above, should we put this more prominently on the table (maybe not immediately but at least mid-term)? 🤔

Would be helpful if someone could collect some necessary base information and references to prepare for this (maybe directly by editing the initial comment for better visibility, pretty sure @vpulim will be ok with this).

[jochem-brouwer]

Linking EIP-225: Clique proof-of-authority consensus protocol

[holgerd77]

Solved by #1032 #1074 #1088 and #1103.

Will close.