WL#2483: Move functions to Storage_engine class

The storage engine specific functions should be made part of the new
Storage_engine interface. See
beolw:

Hello, Magnus.
> 
> The patch now looks overall OK except to the callback functions issue.
> I still believe that callback() and register_query_cache_table() 
> should be made functions of to-be-introduced Storage_engine.
> This structure should be in place as soon as Serg has pushed XA into 
> 5.0, which is the matter of the nearest weeks.
> Please create a Sprint WL task for this little leftover and assign it 
> to yourself or to some other member of NDB team: I'm anxious to ensure 
> that this issue is not lost.


Added as a comment to this WL. / Magnus