Zabbix

........S. [ZBXNEXT-3480] double buffering for host inventories

The patch implement a second cache for the host inventories
to be used for update the dynamic inventory data coming from
the various agents.
When the main configuration cache synchronization is started,
three different situations can be applied to the second cache:

1 - If the inventory is new, it will be added to both the main
   and secondary cache and the one in the secondary cache
   will not have data.

2 - If the inventory need to be updated, the one present in the
   secondary cache will be invalidated. It mean that all the
   dynamic data present will be cleared for having the next
   macro resolution be performed either in the main cache, or
   when updated data are stored in the secondary cache and
   thus, in the database.

3 - If an inventory need to be removed, the one in the secondary
   cache will also be removed.

All the static data will be looked up in the primary cache, even
if there is no real distinction in the lookup routine between
dynamic and static data.

All the strings, for the moment, are kept in the same string pool.
This will change once the configuration cache re-organization
will go in.

The new cache have it's own mutex for locking. Lock and unlock
operations are performed through the new LOCK_INVENTORIES
and UNLOCK_INVENTORIES macros and they are also used
during the main configuration cache synchronization.

The function DCconfig_update_inventory_values(), added
in a previous commit, will not only update the secondary cache
and not touch the main cache in any way.

Discussed with Sasha, vso and wiper.