This commit adds ability to check status of cached file descriptor to
make sure that it can be reused.  We inspect last modification time
and number of hard links.  If last modification is changed from the
last validation time, or number of hard links gets 0, we don't reuse
file descriptor.  We also capped upper limit of the cached file
descriptors.  If the limit is reached, we will close file descriptor
which is least recently used, and its usecount is 0.