Memory usage can be reduced more than 2 times as large hashset is stored in shared memory to hold itemtagid->itemid links, those are used only to find itemtagid when updated or deleted, however it is only done in configuration syncer so there is no use for this to be stored in shared memory.

Before ZBX-25455:
item_tag: sql:0.125474 sync:0.144861 sec 46846848 bytes (333936/0/0).

After ZBX-25455:
item_tag: sql:0.110977 sync:0.101308 sec 22883336 bytes (333936/0/0).

After this change:
item_tag: sql:0.085772 sync:0.063910 sec 7637504 bytes (333936/0/0).