Add automatic swap-to-disk support

[jpsamaroo]

This PR implements support for "storage devices", which represent memory, disks, and storage manager algorithms that MemPool can leverage to perform automatic data management. We implement an SimpleRecencyAllocator device which, when assigned to one or more DRefs (via a poolset kwarg), will manage migrating refs from memory to disk (or really between any two devices) as memory fills up, and from disk to memory as data is accessed via poolget.

The SimpleRecencyAllocator isn't intended to be a perfect, all-encompassing storage manager; instead, it's a simple example of how to do memory management in this new scheme. Interested users are encouraged to implement their own storage devices and managers to suit their workload.

This is also intended as a replacement for some of the logic in JuliaParallel/Dagger.jl#289. This PR should be better able to prevent in-memory data and files on disk from lingering for longer than they're needed, and should give a storage manager fine-grained control over data migration decisions. It doesn't currently integrate metrics about data migration speeds and data compression ratios (which Dagger would want to know), but we should be able to easily expose those metrics later on.

Todo:

• Add tests for SimpleRecencyAllocator
• Add MRU support
• Add docs for writing storage devices
• Add tests for CPURAMDevice and SerializationFileDevice
• Ensure LRU promotes data to upper device during read_from_device
• Expose resource queries (available, capacity)
• Test resource queries
• Try to make all storage-related GC finalizers non-yielding
• Clean-up refcounting debug statements
• Add file name pattern callback to SerializationFileDevice
• Test refcounting implementation
• Update API names to use underscores
• Test nested LRU
• Stress-test concurrent access
• Support retain alternative to destroyonevict and test it
• Test multiple leaf devices
• Add Windows disk query method
• Use Base.diskstats if available
• Determine how to support aliased storage resources Out of scope for now
• Ensure that errors propagate correctly and do not hang the system
• Update Add memory and storage awareness JuliaParallel/Dagger.jl#289 for this PR
• Add mechanism to ensure lazy tasks complete during atexit
• Add stream filters to SerializationDiskDevice for compression/encryption/etc.

[jpsamaroo]

This PR is basically good-to-go. I plan to test this with Dagger's DTable to confirm that it can automatically do swap-to-disk when the env. var. JULIA_MEMPOOL_EXPERIMENTAL_FANCY_ALLOCATOR=1.

[jpsamaroo]

I think this will need to change to avoid the implicit assumption that movetodevice! can move data between devices directly; instead, we should generally assume that moving data between devices implies:

• A read from the old device into memory, if necessary
• A delete from the old device
• A write from memory to the new device
• An optional delete from memory