Fluffy shouldn't allow running multiple instances from the same data directory

[web3-developer]

Fluffy doesn't fail or show any error message when starting a second Fluffy instance pointing to the same data directory. The --data-dir parameter is also not a required parameter and will use the default directory if not specified therefore it is rather easy for a user to not supply it and mistakenly run multiple instances pointing to the same location.

We should create a lock file on startup in the data directory so that if a second instance is started pointing to the same location it can read the lock file and shutdown with a error message to let the user know the root cause.