lvconvert: support conversion to thin volume

lvconvert: support conversion to thin volume

Update pool conversion function to handle also conversion of
thick LV to thin LV by moving thick LV into thin pool data LV
and creating fully provissioned thin LV on top of this volume.

Reworking existing conversion to use insert_layer_for_lv co
the uuid is now kept with thin-pool - this should however not
really matter as we are doing full deactivation & activation cycle.

With conversion to thin LV user can use same set of arguments
to set chunk-size.

TODO: add some smart code to decide best values for chunks sizes.