[th/libnm-active-connection-delay-ready]

Libnm's NMClient cache has a notion of objects being ready/visible.

This code is utterly complicated, and the reasons are hard to comprehent. On the other hand, I wrote it in one afternoon, so it probably could be rewritten with a reasonable effort. Anyway, I think the code works well in practice. Here is a small tweak to delay declaring NMActiveConnection as ready.

This fixes that when nm_client_add_add_activate*() completes, ac.get_connection() does not return NULL but the initialized NMRemoteConnection. This can be easily reproduced by looking at ac.get_connection() from the callback of the add-and-activate call.