JIT-unspill: fix potential deadlock

[madsbk]

Fixes a deadlock where multiple threads accesses ProxifyHostFile.maybe_evict() but none of them can acquire both the ProxifyHostFile lock and the ProxyObject lock simultaneously.

Should fix_ rapidsai/gpu-bdb#162 (comment)

[codecov-io]

Codecov Report

Merging #501 (a755803) into branch-0.18 (32d9d33) will increase coverage by 0.93%.
The diff coverage is 95.01%.

@@               Coverage Diff               @@
##           branch-0.18     #501      +/-   ##
===============================================
+ Coverage        90.42%   91.35%   +0.93%     
===============================================
  Files               15       18       +3     
  Lines             1128     1435     +307     
===============================================
+ Hits              1020     1311     +291     
- Misses             108      124      +16     

Impacted Files	Coverage Δ
dask_cuda/cli/dask_cuda_worker.py	96.92% <ø> (ø)
dask_cuda/cuda_worker.py	78.75% <75.00%> (+1.73%)	⬆️
dask_cuda/device_host_file.py	90.90% <80.00%> (-7.96%)	⬇️
dask_cuda/get_device_memory_objects.py	89.04% <89.04%> (ø)
dask_cuda/proxify_device_objects.py	91.83% <91.83%> (ø)
dask_cuda/proxy_object.py	89.80% <95.04%> (+2.00%)	⬆️
dask_cuda/explicit_comms/comms.py	99.02% <100.00%> (+0.01%)	⬆️
dask_cuda/explicit_comms/utils.py	100.00% <100.00%> (ø)
dask_cuda/local_cuda_cluster.py	81.17% <100.00%> (+0.68%)	⬆️
dask_cuda/proxify_host_file.py	100.00% <100.00%> (ø)
... and 5 more

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[pentschev]

Note for future reading: hostfile.lock is an RLock, so this block is ok.

[pentschev]

LGTM, thanks @madsbk !

[pentschev]

@gpucibot merge