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
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