Use the same file handle in file system `getBlob`

[gaul]

Previously getBlob lazily returned a ByteSource which could return a different content length than the blob metadata. Instead eagerly open a FileInputStream for consistency. Note that this has some consequences for LocalBlobStore.getBlob when used with ranges.

[gaul]

I am still thinking about the range issue.

[gaul]

Abandoning this since S3Proxy's filesystem-nio2 provider does what I need: https://github.com/gaul/s3proxy/issues/697.