This fixes 2 related issues:
* Now that the bor consensus engine is required for queries it can't be
created based on the pretense of a db directory, but must be based on
chain config read from the db. Using the DB presence causes Bor to get
instantiated for non bor chains which breaks.
* At the moment eth_calls on a remote daemon don't check Bor headers
prior to calling the EVM code as it was just using a fake ETHash
instance - which performs ETH header validation only.
The current version is mostly working but needs adapting to perform lazy
initialization of the engine.
