Disk io library: add missing const qualifier to doubly linked list accessors.
Client library: implement i-node cache entry integrity validation, allow to specify i-node re-validation scan length. Fix KfsClientImpl::StatSelf() by handling possible i-node cache entry deletion from cache re-validation invoked from KfsClientImpl::GetPathComponents(), and KfsClientImpl::NewFattr() by setting current cache entry pointer to 0, if it gets delete as the result of re-validation, or cache "trimming".