PilotUUID non-uniqueness can break Dirac and DiracX

I think it is a known issue, but if we can add to the fixes of V9..

Currently, Dirac `PilotAgentDB` (as well as DiracX) has a `PilotID` primary key, an `Integer`. Most of the time, we use `PilotJobReferences` instead of `PilotID`, and we even translate the `PilotJobReference` into `PilotID`:

```py
def deletePilot(self, pilotRef, conn=False):
"""Delete Pilot with the given reference from the PilotAgentsDB"""

if isinstance(pilotRef, str):
pilotID = self.__getPilotID(pilotRef)
else:
pilotID = pilotRef

return self.deletePilots([pilotID], conn=conn)
```

With the example above, we will delete a pilot with a `pilotReference`. If for whatever reason there is a duplicate (two pilots with the same reference), we would delete one of them without being sure of which one.

This is an issue later on with DiracX with secrets, because:

- if there is a duplicate by error, one of the pilots won't have a secret
- we could have a race condition where the original pilot doesn't have a secret, and the "malicious" one could obtain a secret
- currently a pilot only knows its reference, not its ID (it's own identity is based on a non-unique string, it is not an identity anymore)

Changes:

```sql
-- From
`PilotJobReference` VARCHAR(255) NOT NULL DEFAULT 'Unknown',
-- To
`PilotJobReference` VARCHAR(255) UNIQUE NOT NULL DEFAULT 'Unknown',
```