fuse: changed file read implementation to avoid OOM (#620)
fuse: changed file read implementation to avoid OOM (#620)

Changed file read implementation from ReadAll() to a Handle to avoid OOMing

We don't have automated tests for this but I verified this by restoring
13GB file over fuse and memory usage never exceeded 400MB.
kopia/kopia,kopia/kopia,kopia/kopia,kopia/kopia