Add "affinity" tracking to nodes and conductors

Add "conductor_affinity" column to nodes table, containing a reference to the
`id` of the conductor service (not its hostname) that has most recently
performed some action which could require local state to be maintained
(eg, built a PXE config, or started a SOL session).

Using the `id` as a foreign key necessitates not deleting conductors
when unregistering them, but instead marking them offline. This also
helps in determining if a conductor service was only restarted (though
this patch does not implement graceful shutdown).

Thus, this patch also adds an "online" boolean column to the conductors
table to track whether a conductor is on- or offline, and updates
the register and unregister methods to use that field transparently.
It may be noted that this does not change the behavior of
register_conductor or unregister_conductor, though an optional
"update_existing" parameter has been added to register_conductor. This
replaces a DELETE query with an UPDATE query instead.

Co-Authored-By: David Shrewsbury <shrewsbury.dave@gmail.com>
Co-Authored-By: Lucas Alvares Gomes <lucasagomes@gmail.com>

Related-bug: #1279331
Change-Id: I8e8b5cc00fc9f565ad2fb442e9a26077342e0a25
Merge "Add "affinity" tracking to nodes and conductors"