What we want to do here is keep separate caches of system and
agent secrets.  For system secrets, we cache them because NM
periodically clears secrets using nm_connection_clear_secrets() to
ensure they don't stay around in memory, and that transient secrets
get requested again when they are needed.  For agent secrets, we
only want them during activation, but a connection read from disk
will not include agent secrets becuase by definition they aren't
stored in system settings along with the connection.  Thus we need
to keep the agent/transient secrets somewhere for the duration of
the activation to ensure they don't get deleted.

This removes the copy-back hack in update_auth_cb() which copied
agent/transient secrets back into the connection over top of the
transient secrets that had been copied back in
nm_settings_connection_replace_settings().  No reason to copy
them twice if we keep an agent/transient secrets hash and do
the right thing with it.