Use .lock file for data directory #1989
Description
When playing with two XUDs on a local regtest network, I noticed that they were using the same database and node public key. This is not a critical issue since I don't think that this could cause data corruption of any files, but it is an inconvenience that XUD does not prevent that from happening. There is no use case for using the same data directory for two nodes at the same time, so we should not let XUD do that since it could cause some weird behaviour.
What I propose is creating an empty .lock file for the data directory (or maybe node public key and database file) on startup and removing that file when XUD shuts down. When XUD detects that such a file exists, it should abort the startup and log that there might be an XUD running with that data directory already