Add support for replaying from renamed block log

Previously when node was preparing replay and copying block log file, it does
not care about block log name. If block log was named bad_block_log, then it
was copied with that name to SomeNode/blockchain directory. But it caused
problem, because node requires to have block log with name "block_log".

Now this name is enforced. During copying, destination file name is always set
to valid "block_log" name.

It allows to replay from renamed block log with syntax:
```
node.run(replay_from="/path/to/block_logs/block_log_5m")  # Note custom block log name
```