thin: add common pool functions

thin: add common pool functions

Move common functions for lvcreate and lvconvert.

get_pool_params() - read thin pool args.
update_pool_params() - updates/validates some thin args.

It is getting complicated and even few more things will be
implemented, so to avoid reimplementing things differently
in lvcreate and lvconvert  code has been splitted
into 2 common functions that allow some future extension.