Only perform DAO hard fork challenge if Aleth's top block is >= DAO hard fork block

[halfalicious]

Aleth contains a DAO hard fork challenge which it issues to potential peers in order to avoid wasting time syncing to ETC nodes. Additional details on the motivation for this challenge are described here: #5540 (comment)

The way the challenge works is that Aleth requests the DAO hard fork block from a potential peer and validates the extradata section of the block. If the peer is an ETC node, the hard fork block won't contain the expected extradata data and Aleth will avoid peering with the node.

The block is requested here:

aleth/libethereum/BlockChainSync.cpp

Lines 204 to 209 in 6de1e88

	// Before starting to exchange the data with the node, let's verify that it's on our chain
	if (!requestDaoForkBlockHeader(_peer.id()))
	{
	// DAO challenge not needed
	syncPeer(_peer.id(), false);
	}

The challenge is verified in BlockChainSync::onPeerBlockHeaders:

aleth/libethereum/BlockChainSync.cpp

Lines 444 to 453 in 6de1e88

	if (m_daoChallengedPeers.find(_peerID) != m_daoChallengedPeers.end())
	{
	if (verifyDaoChallengeResponse(_r))
	syncPeer(_peerID, false);
	else
	m_host.disablePeer(_peerID, "Peer from another fork.");
	m_daoChallengedPeers.erase(_peerID);
	return;
	}

However, this challenge prevents us from syncing blocks with ETC nodes from before the DAO hard fork block. An optimization would be to only perform the DAO hard fork challenge if our top block is >= the DAO hard fork block (1920000).