activation: use cmd pending mem for pending_delete

activation: use cmd pending mem for pending_delete

Since we need to preserve allocated strings across 2 separate
activation calls of '_tree_action()' we need to use other mem
pool them dm->mem - but since cmd->mem is released between
individual lvm2 locking calls, we rather introduce a new separate
mem pool just for pending deletes with easy to see life-span.
(not using 'libmem' as it would basicaly keep allocations over
the whole lifetime of clvmd)

This patch is fixing previous commmit where the memory was
improperly used after pool release.