dmeventd_thin: new logic for calling commands

dmeventd_thin: new logic for calling commands

For more advanced support we need to ensure better logic for calling
external much more advanced script for maintanance of thin-pool.

So this new code ensures:

When thin-pool data or metadata is bigger then 50%,
then with each 5% increment, action is called.
This is independent from autoextend_threshold.
This action always happens when thin-pool is over threshold,
(so no action when it's exactly i.e. 60%).
The only exception is 100% full thin-pool - which invokes 'last'
action.

Since thin-pool occupancy may change also downward, code needs
to also handle possibly reduction of occupancy  of thin-pool.
So when usage drop from 90% to 50%, thin-pool will start to call
again action when it will pass 55% threshold.

This give external commands lot of option i.e. to call 'fstrim'
before actual resize is needed.