*: reorganise indexation methods
02011d8e (immediateReindexObject: use super user to reindex script, 2015-12-14) did not apply for inventory, because they were overloading immediateReindexObject. Introduce a new level, _immediateReindexObject that will hold the actual reindexing logic.
Previously this method was using PortalContent.reindexObject which was monkey patched, to make things less complex and more future proof, move the monkey patch to a method on base class.
This also drops alternateReindexObject on BalanceTransaction, because it is already defined in Inventory