src/osd/: add ECBackend stubs

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd_types: introduce cpg_t, pg_shard_t

Signed-off-by: Samuel Just <sam.just@inktank.com>
working
working
spg_t

Signed-off-by: Samuel Just <sam.just@inktank.com>