Add SPARQL optimize bus

[rubensworks]

This bus should allow queries (in SPARQL algebra) to be rewritten by actors on its bus.

Actors could use this bus to optimize certain query types, or to modify certain operations so that certain specific actors can evaluate them.

[rubensworks]

A separate bus is not really needed anymore.

Actors wanting to optimize certain queries could simply register to the query operation bus, listen to operations fulfilling the optimization requirements, optimizing those, and pushing those optimized operations to the same bus again.

This is even more flexible than creating a separate bus for this, as all operations could be optimized using this mechanism.