getting one data chunk back. limiting, via config, how big the data chunk is

Signed-off-by: Alexander Ginzburg <alex@eightbits.com>
implementation of client and server sides write()
 re-implementation of file tree: data_root/<data_chunk> now has 'dprfs.next' extended attribute

Signed-off-by: Alexander Ginzburg <alex@eightbits.com>