core/dbus: do not block the manager on GetId during bus (re-)connection

core/dbus: do not block the manager on GetId during bus (re-)connection

bus_init_api() issued a synchronous GetId call on every API bus
(re-)connection to decide whether saved subscription state could be
coldplugged onto the new connection.

If the D-Bus socket unit is listening while the message bus daemon
behind it is gone, connect() succeeds against the socket backlog but
nothing answers the authentication handshake. The synchronous call
then blocks PID 1 for BUS_AUTH_TIMEOUT (90 seconds by default), and
queued bus operations can trigger repeated reconnection attempts.
This was observed during shutdown as roughly 15 minutes of teardown
progressing only in 90-second intervals.

Query the instance ID asynchronously on every connection. Defer API
setup until the reply is processed, so saved subscriptions are
validated and coldplugged before new subscription requests can arrive.
If the query cannot be queued or its reply is invalid, discard the
unvalidated state and expose the API without blocking the manager.

Reset the live bus ID on every connection and serialize pending bus ID
and subscription state across reload and reexec. During daemon-reload,
preserve state that was already awaiting the asynchronous reply while
discarding the duplicate state produced by the reload itself.

Also remove the now-unused synchronous bus_get_instance_id() helper.