Important: #92356 - DataHandler performance improvements

See forge#92356


The core DataHandler is the central backend heart of the system to persist state changes in the database whenever editors change elements.

Some changes have been applied to reduce the database query load performed by the DataHandler and its related classes. Latest changes especially dropped a number of useless queries and php operations when relations are handled. Depending on the handled structure, the DataHandler executes up to 30% less queries than before. More improvements continue to happen and will be ported to v10 if possible.