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
This is to fix an issue with resource usage if the db.size.limit is
increased from its default setting of 2TB. This is applied to the chain
DB, but should not be used on the consensus DB which has smaller data
requirements. Expanding both DBs results in excessive RAM being reserved
by the underlying OS.
