[c-lightning] Add secondary persistence for sqlite3 replication

[bboerst]

Closes #103

This PR adds a secondary persistent storage option that mounts to /root/backup. The intended purpose is to use built-in CLN db replication by adding configuration like:

wallet=sqlite3:///root/.lightning/bitcoin/lightningd.sqlite3:/root/backup/lightningd.sqlite3

This way the main lightningd.sqlite3 db will be handled by persistence: and the replica will be handled by persistenceBackup:.

In my setup, I'm using local-path-provisioner for the main pvc and an nfs-external-provisioner for the backup storage. This way the db is replicated to an NFS share and periodically backed up. It's still not ideal because the replica on NFS is getting hot-backed-up, but in this scenario 3 different things will all have to fail:

1. Main db failure
2. Replica failure
3. Nightly backup failure

The odds of all 3 failing is not likely, but this will suffice until a more elegant backup strategy can be implemented in the future. Maybe some kind of sidecar running sqlite3 .backup on a cron or something.