Chunk server: implement chunk "writable" check; pass "buffered io" flag to "writable directory" check to determine if the host file system supports direct io or not; implement per chunk directory buffered io setting in order to make ram fs that doesn't support direct io coexist with xfs (or other file systems that support direct io). This, in turn, is need to support ram fs based storage tiers along with conventional xfs flush or conventional spinning disks based storage tiers.