Fix $reset parameter for load functions

I just realized that, currently, the $reset parameter for the search_api_(server|index)_load(_multiple)? functions doesn't have any function. That's because these function now internally use the entity_load_multiple_by_name() function, which for some reason doesn't provide a $reset parameter like all other load functions.
The fix is simply to inline that function's body into our two functions, which should also give a slight performance improvement (though hardly a noticable one).