[EXPERIMENT] Change the default tuning to small

Setting the default tuning to large improves performance, but at the same time it also makes the memory consumption to go up which is only amplified by the way how BIND 9 works with memory - the net result is higher memory fragmentation in the default system memory allocator and visible increase in RSS.

Switch the default tuning to small as it was in the 9.11, the default is sufficient in most of the BIND 9 installations.

Closes #3663 (closed)