Syncing z2 nodes delays API calls

[rrw-zilliqa]

Whilst a zq2 node (run in this case by z2) is syncing, you get long delays in some API responses since the node is spending most of its time validating incoming blocks. To show this:

• Start a 4-node z2 network and run it for a few hundred (empty) blocks
• Stop node 1
• Remove its data directory
• Restart it
• Attempt to send an eth_syncing API call to it
• Observe that the node doesn't reply for a few seconds because it's too busy syncing.