Introduce installation IDs

[io7m]

This is an installation-specific UUID value, shared across replicas and configured in the configuration file (and written to the database on startup), that uniquely identifies an installation.

If an application with ID x tries to connect to a database that has ID y (where x ≠ y), the connection will be rejected with an informative error.

If people print stock identifiers as QR codes, the QR code will include the installation ID.