client API: add async path; fix deadlock for vo_libmpv users

In commit 065c307e8e7db, I broke everything. It seemed like a nice idea,
but it explicitly broke an assumption vo_libmpv were explicitly allowed
to make: that observing properties does not lock the core. The commit
did just that and locked the core for property updates. This made for
example mpv's own OSX backend freeze (it uses vo_libmpv for convenience
to make up for Apple's incredibly broken OpenGL shit).

I don't want to revert that commit just because vo_libmpv's design is
horrible. So instead add an optional asynchronous path, that is only
used if vo_libmpv is in use (best idea ever?).

Interestingly, this isn't so hard. It adds about 90 lines of code, which
are only run on OSX and libmpv users, so I don't have to care about the
crashes and weird behavior this might cause. It even worked on the first
try except for a quickly fixed memory leak. The code path can be tested
anywhere by just turning the uses_vo_libmpv condition into always true.

The atomic is out of laziness. Saves some thinking how to get around the
locking order.

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