Enforce clean startup when importing from snapshot

[fridrik01]

When starting Lotus from snapshot its easy to forget to manually cleanup the .lotus/datastore folder before running the --import-snapshot command. This may result in issues/errors that are difficult to debug.

Proposal

We should check whether the .lotus/datastore folder exists when starting lotus --import-snapshot. If the folder exists, we should do one of:

1. Always exit Lotus with a non-zero exit code and error message that user needs to delete the .lotus/datastore and run the import again.
2. Always automatically cleanup the .lotus/datastore and continue the import.
3. Always ask user (Y/N) whether Lotus should cleanup the .lotus/datastore folder and then (if Y) continue the import, or (if N) exit Lotus with non-zero exit code. Also introduce a --force-import-cleanup flag that if set will automatically select Y so this could be run non-interactively from a script.

Discussion

a) Which (1,2,3) is the best
b) Its my understanding that .lotus/datastore is the only folder that needs to be deleted. However if we will be doing a cleanup of the .lotus/datastore should we also delete other folders like .lotus/sqlite although its not required .
c) For context, this enhancement request started from this slack thread.