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.
This introduces _experimental_ block execution run by embedded Silkworm
API library:
- new command-line option `silkworm.path` to enable the feature by
specifying the path to the Silkworm library
- the Silkworm API shared library is dynamically loaded on-demand
- currently requires to build Silkworm library on the target machine
- available only on Linux at the moment: macOS has issue with [stack
size](https://github.com/golang/go/issues/28024) and Windows would
require [TDM-GCC-64](https://jmeubank.github.io/tdm-gcc/), both need
dedicated effort for an assessment
