Implement cache size for CachingFileSystem

[ianthomas23]

This PR implements retrieving the cache size of CachingFileSystem. The latest size is stored as a private attribute which is invalidated on any write to the cache and only recalculated if necessary when cache_size() is called. If multiple storage locations are specified it only considers the last one, i.e. the writable one.

This is a necessary precursor to providing a cache size limit (#510).

[ianthomas23]

I've temporarily removed one of the checks for filecache on Windows as when you delete a cached file the parent directory gets larger not smaller 🤷

The remaining test that is failing on Windows is a blockcache one that does not allow deletion of a file that it claims is still in use. I don't yet know if this is something wrong with Windows or fsspec.

[martindurant]

does not allow deletion of a file that it claims is still in use. I don't yet know if this is something wrong with Windows or fsspec.

Windows does like to keep locks on files...

[ianthomas23]

If I remove the problematic asserts related to file locking for blockcache on Windows then there isn't really anything left worth testing, so I've just skipped that test on Windows. This isn't ideal, but I would rather have the functionality available on posix systems than not at all.

[martindurant]

This isn't ideal, but I would rather have the functionality available on posix systems than not at all.

Agreed - but I expect it works fine in practice, and at most results in warnings when closing the interpreter. "Sparse" files are not a thing on windows anyway, so you only get part of the benefit of blockcache.