Remove distribution set from file path on storage
Previously the data and header files where stored in a directory whose
name is the distribution set. In a layout 0 there is only 24 distribution
for 4 servers, but in layout 1 with 16 servers the number of distribution
set is 2 E+13 !!! Storing files in a such a named directory does not scale.
Now the path on a storage is :
<root_path>/device/<type>/<slice>/FID
<root_path> is the root path of the cid/sid
<type> is eitheir :
  - bins_0 for non spare bins files
  - bins_1 for spare bins files
  - hdr_0 for non spare header files
  - hdr_1 for spare header files
<slice> a slice number. May be ommitted when only one slice exist