it doesn't matter, because we don't use it, but for the sake of readability

some code is expecting things to happen in that order...

... in case the global is removed from the registry before
the initial augment completes

When a new global is created, it is not certain
if the registry global event or the proxy bound event will
be fired first. In order to make sure we associate all
proxies to their WpGlobals correctly, we now wait a core sync
before exposing globals to the object managers, so that in case
the implementation proxy receives the bound event after the
registry creates the WpGlobal, we can make sure to use this
proxy instead of constructing a new one through the object managers

There are 3 kinds of WpProxy objects:
 * the ones that are created as a result of binding a global
   from the registry
 * the ones that are created as a result of calling into a remote
   factory (wp_node_new_from_factory, etc...)
 * the ones that are a local implementation of an object
   (WpImplNode, etc...) and are exported

Previously the object manager was only able to track the first kind.
With these changes we can now also have globals associated with
WpProxies that were created earlier (and caused the creation of the global).
This saves some resources and reduces round-trips (in case client
code wants to change properties of an object that is locally
implemented, it shouldn't need to do a round-trip through the server)

+ use the pw_proxy API to find the bound id instead
of relying on WpGlobal

This has the advantage that it works also for exported
objects and for objects that have been created by calling
into a remote factory (such as the link-factory), so we can
now know the global id of all proxies, not only the ones
that have been created by the registry.

Otherwise, if the object manager is destroyed while a sync is in progress,
we get an invalid 'self' pointer on the callback later, which is being
called regardless

There is a bit more work that should be done in the core to avoid leaking
this ref in case pipewire disconnects before the sync is completed