LU-10070 lod: SEL: interoperability support

LU-10070 lod: SEL: interoperability support

Add a new SEL magic for storing SEL components on disk.
It is never gets out of LOD, converted on read/write to COMP_V1.
A the result, old MDS is not able to open SEL files.
At the same time old clients are able to work with existing files
seamlessly. Old clients still lacks lustre utils support, thus not
possible to create new SEL files, etc.

Cray-bug-id: LUS-2528
Signed-off-by: Vitaly Fertman <c17818@cray.com>
Change-Id: Ib3f0b1402cd920e56beaad78a74da485bd7ad342
Reviewed-on: https://review.whamcloud.com/35144
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>