client: Replace the singleton zombie with bespoke zombies

Using the singleton zombie object doesn't allow us to posthumously retain
object interface information, which makes it difficult to properly inter
future events destined for the recently deceased proxy.

Notably, this makes it impossible for zombie proxy destined file
descriptors to be properly consumed.

When we create a proxy, we now create a zombie-state object to hold
information about the file descriptors in events it can receive. This
will allow us, in a future patch, to close those FDs.

[daniels: Split Derek's patch into a few smaller ones.]

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>