Zabbix

........S. [ZBXNEXT-3480] store host inventory in cache

Store the full hopst inventory in configuration cache. At the
moment, the implementation use the cache string pool for
storing all the inventory data for every host.
Inventory structures are being retrieved, compared and
stored properly in cache.

No changes on the configuration cache structures are
performed for now. This is because I want to be sure that
everything works properly before touching it.

This diff is the preparation for expanding inventory macros
from the configuration cache and not rely on the database
for that.