Add racefree way to add a new unique data point

Add racefree way to add a new unique data point

Our schema gathers together data related to a service by using an ID
column. This column cannot be unique or a primary key as the ID is
repeated for each key/value pair in the datum group.

Use a unique identifier to make sure we can let dqlite generate a new
ID internally and then find out wat it is as race-free as possible.

We keep this method in the data module so it can be changed later
without affecting application logic.

Signed-off-by: Simo Sorce <simo@redhat.com>