WIP fcache

Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>
Simple LRU file cache

Simple file cache with LRU. The cache holds a maximum number of files (not
based on size). Heap queue is used for the LRU cleanup. The entries of the
cache are removed until cleanup().

Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>