This PR extends the `nimbus import` command to also allow reading from
era files - this command allows creating or topping up an existing
database with data coming from era files instead of network sync.
* add `--era1-dir` and `--max-blocks` options to command line
* make `persistBlocks` report basic stats like transactions and gas
* improve error reporting in several API
* allow importing multiple RLP files in one go
* clean up logging options to match nimbus-eth2
* make sure database is closed properly on shutdown
The existing vmState tracing is plugged into chronicles, at the TRACE
level, to facilitate state test debugging.
Some useful chronicles defines are added to "tests/nim.cfg" to simplify
the compile-and-run command for individual tests.