Meta server: add debug trace message to display all relevant chunk sever properties then chunk server is no longer considered a placement
candidate. Reduce writable chunk size over estimation, as the chunk server now has the "virtual" space reservation to support host file
systems with no space reservation / pre-allocations.
Chunk server: use posix_fallocate() on linux instead of xfs specific space reservation api. Implement the logic to see if the space allocation is really supported or not in directory checker, by attempting to do file space reservation to see if the call succeeds, and if it does, then use stat to see if the file on disk size reflects the space reservation or not. The check is performed with file opened with O_DIRECT flag is direct io is supported and enabled to handle the case where the host file systems do not support space reservation with direct io, like ext3 for example.