ETH.getBlockByHash goes into infinite loop if block not found

[acolytec3]

In the course of working on #710, I discovered that our client.ETH.getBlockByHash implementation can go into an infinite loop if we manage to index a blockhash <> block number mapping but where the block isn't stored correctly in the DB. At the moment, this appears to mainly be an issue when the node doesn't have any external peer connections.

The issue occurs in this code branch. We don't find the block by the hash, but we do have a mapping of the block hash to block number so we try looking it up by number. If we don't find it by number, but do have the number to hash mapping, we then try to look it up by hash again.