Changing a partial harvest next date end up saving the whole harvest hierarchy

On Definitions-edit-selective-harvest.jsp, when only assigning a new next date to a partial harvest, leaving all other fields untouched, and then clicking the save button, we go through the method SelectiveHarvestUtils#updateHarvestDefinition, which saves every objects of the harvest hierarchy in the DB, even though nothing but the nextDate has been modified. This ends up in very bad performance on large harvests, and block BnF curators (the connection pool gets saturated).