thin: use lv_status_thin and lv_status_thin_pool

thin: use lv_status_thin and lv_status_thin_pool

Introduce structures lv_status_thin_pool and
lv_status_thin  (pair to lv_status_cache, lv_status_vdo)

Convert lv_thin_percent() -> lv_thin_status()
and  lv_thin_pool_percent() + lv_thin_pool_transaction_id() ->
lv_thin_pool_status().

This way a function user can see not only percentages, but also
other important status info about thin-pool.

TODO:
This patch tries to not change too many other things,
but pool_below_threshold() now uses new thin-pool info to return
failure if thin-pool cannot be actually modified.
This should be handle separately in a better way.