fs: Re-implement object layer to remember the fd (#3509)

This patch re-writes FS backend to support shared backend sharing locks for safe concurrent access across multiple servers.

cmd/posix.go

@@ -652,7 +652,6 @@ func (s *posix) createFile(volume, path string) (f *os.File, err error) {
 // PrepareFile - run prior actions before creating a new file for optimization purposes
 // Currently we use fallocate when available to avoid disk fragmentation as much as possible
 func (s *posix) PrepareFile(volume, path string, fileSize int64) (err error) {
-
 	// It doesn't make sense to create a negative-sized file
 	if fileSize <= 0 {
 		return errInvalidArgument