api key: include reference count for garbage collection of auto-generated api-keys

[pcrespov]

Some auth tokens are created automatically (e.g. )

We need to enhance the interface of the api_keys plugin in the webserver

• creation/deletion interface can include optionally reference counting
• GC auto-delete keys when reference count reaches zero

Add the possibility of having a round-robin on a set of api-keys

[pcrespov]

NOTE: see upsert in get_or_create. Can add a reference counter there