netbox: don't fire on_connect() at schema update

There was a bug that netbox at any schema update called
on_connect() triggers. This was due to overcomplicated logic of
handling of changes in the netbox state machine. On_connect() was
fired each time the machine entered 'active' state, even if its
previous states were 'active' and then 'fetch_schema'. The latter
state can be entered many times without reconnects.

Another bug was about on_disconnect() - it could be fired even if
the connection never entered active state. For example, if its
first 'fetch_schema' has failed.

Now there is an explicit flag showing the machine connect state.
The triggers are fired only when it is changed, on 'active' and on
any error states. Intermediate states (fetch_schema, auth) do not
matter anymore.

Thanks @mtrempoltsev for the initial investigation and a draft
fix.

Closes #4593

(cherry picked from commit d56d869a)