Accessing beyond the end of the file now removes the file from the cache
because it is assumed to be truncated. Usually, this means that the data
is fetched directly from the backend instead.
This commits adds rudimentary support for a cache directory, enabled by
default. The cache directory is created if it does not exist. The cache
is used if there's anything in it, newly created snapshot and index
files are written to the cache automatically.
greatroar