*: 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