report: add lv_ancestors and lv_descendants reporting fields

report: add lv_ancestors and lv_descendants reporting fields

Show full chain of ancestors and descendants for snapshots
(both thick and thin - in case of thick, the "ancestor" field
is actually equal to "origin" field as snapshots can't be
chained for thick snapshots).

These fields display current state as it is, they do not
display any history! If the snapshot chain is broken in
the middle, we don't report the historical origin (this
is going to be a part of another patch and a different
set of fields or just a switch for existing fields to
show ancestors and descendants with history included).

For example:

(origin --> snapshot)

lvol1 --> lvol2 --> lvol3 --> lvol4
              \
                --> lvol5 --> lvol6 --> lvol7 --> lvol8

$ lvs -o name,pool_lv,origin,ancestors,descendants vg
  LV    Pool Origin Ancestors                     Descendants
  lvol1 pool                                      lvol2,lvol3,lvol4,lvol5,lvol6,lvol7,lvol8
  lvol2 pool lvol1  lvol1                         lvol3,lvol4,lvol5,lvol6,lvol7,lvol8
  lvol3 pool lvol2  lvol2,lvol1                   lvol4
  lvol4 pool lvol3  lvol3,lvol2,lvol1
  lvol5 pool lvol2  lvol2,lvol1                   lvol6,lvol7,lvol8
  lvol6 pool lvol5  lvol5,lvol2,lvol1             lvol7,lvol8
  lvol7 pool lvol6  lvol6,lvol5,lvol2,lvol1       lvol8
  lvol8 pool lvol7  lvol7,lvol6,lvol5,lvol2,lvol1