Add workaround for too expensive optimisation for SkyTec.
Add workaround for too expensive optimisation for SkyTec.

The details are in bug #2595.  Using debug=65536 we can now
(temporarily) disable the expensive or-style optimisation of selection
push-downs.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb