Errors freeing memory atexit

At process exit, command queues and contexts are collected before allocations are, resulting in crashes:

```
From worker 13: [264837] signal 11 (1): Segmentation fault
From worker 13: in expression starting at none:0
From worker 13: clFinish at /home/tim/.julia/artifacts/c99d19d2f6dd75af3e997c4ecc4ac85e410cb7a7/lib/libOpenCL.so (unknown line)
From worker 13: macro expansion at /home/tim/Julia/pkg/OpenCL/lib/cl/libopencl.jl:703 [inlined]
From worker 13: #113 at /home/tim/Julia/pkg/OpenCL/lib/cl/api.jl:29 [inlined]
From worker 13: #1 at /home/tim/Julia/pkg/OpenCL/lib/cl/libopencl.jl:13 [inlined]
From worker 13: retry_reclaim at /home/tim/Julia/pkg/OpenCL/lib/cl/api.jl:49
From worker 13: check at /home/tim/Julia/pkg/OpenCL/lib/cl/libopencl.jl:10 [inlined]
From worker 13: clFinish at /home/tim/Julia/pkg/OpenCL/lib/cl/api.jl:34 [inlined]
From worker 13: finish at /home/tim/Julia/pkg/OpenCL/lib/cl/cmdqueue.jl:62 [inlined]
From worker 13: synchronize at /home/tim/Julia/pkg/OpenCL/src/memory.jl:29 [inlined]
From worker 13: #10 at /home/tim/Julia/pkg/OpenCL/src/memory.jl:149 [inlined]
From worker 13: device! at /home/tim/Julia/pkg/OpenCL/lib/cl/state.jl:152
From worker 13: free at /home/tim/Julia/pkg/OpenCL/src/memory.jl:143
From worker 13: unknown function (ip: 0x74d0d7939ab2)
From worker 13: release at /home/tim/.julia/packages/GPUArrays/Mot2g/src/host/abstractarray.jl:42
From worker 13: unsafe_free! at /home/tim/.julia/packages/GPUArrays/Mot2g/src/host/abstractarray.jl:100 [inlined]
From worker 13: unsafe_free! at /home/tim/.julia/packages/GPUArrays/Mot2g/src/host/abstractarray.jl:115
From worker 13: unknown function (ip: 0x74d0d79f6a92)
From worker 13: run_finalizer at /cache/build/builder-demeter6-3/julialang/julia-release-1-dot-11/src/gc.c:299
From worker 13: jl_gc_run_finalizers_in_list at /cache/build/builder-demeter6-3/julialang/julia-release-1-dot-11/src/gc.c:389
From worker 13: run_finalizers at /cache/build/builder-demeter6-3/julialang/julia-release-1-dot-11/src/gc.c:435
From worker 13: ijl_atexit_hook at /cache/build/builder-demeter6-3/julialang/julia-release-1-dot-11/src/init.c:299
```