thin: activate layer pool aas read-only LV

thin: activate layer pool aas read-only LV

When lvm2 is activating layered pool LV (to basically keep pool opened,
the other function used to be 'locking' be in sync with DM table)
use this LV in read-only mode - this prevents 'write' access into
data volume content of thin-pool.

Note: since EMPTY/unused thin-pool is created as 'public LV' for generic
use by any user who i.e. wish to maintain thin-pool and thins himself.
At this moment, thin-pool appears as writable LV.  As soon as the 1st.
thinLV is created, layer volume will appear is 'read-only' LV from this moment.