cache: introduce rudimentary CacheFile API

Throughout the existing cache-related code there's constant checking
if compression is active or not.

This commit introduces a CacheFile ADT with the intention of
replacing all the open coded stuff with simple calls into
rmdCacheFile* functions operating on CacheFile instances.

The management of split up cache files has also been implemented
behind this API, so the reads and writes will transparently
handle the split files.  These have been named "chapters" in new
code.

No callers have been changed, this only adds the new facility.  A
subsequent commit will migrate things over.

0 files changed, 0 insertions, 0 deletions