process: ensure uvfinalize and _uv_close_cb are synchronized #44476
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There appeared to be a possibility they could race and the data field
might already be destroyed before we reached the close callback,
from looking at the state of the program when reproducing #44460.
This is because the uv_return_spawn set the handle to NULL, which later
can cause the uvfinalize to exit early (if the finalizer gets run on
another thread, since we have disabled finalizers on our thread). Then
the GC can reap the julia Process object prior to uv_close cleaning up
the object. We solve this by calling disassociate_julia_struct before
dropping the reference to the handle. But then we also fully address any
remaining race condition by having uvfinalize acquire a lock also.
Fixes #44460
(added backport labels, since in quick testing, it seems to apply cleanly to those branches)