Initial attempt at getting R-Tree queries to work using a priority queue.
This check-in compiles, but R-Trees do not work well.  And there are
debugging printf()s left in the code.  This is an incremental check-in.
Bug fixes to the priority-queue implementation for R-Trees.  Improved tracing
capability.  Some queries work now, but still many problems.
Fix a bug in rowid=? query handling.  More problems remain.
TCL tests now all pass.
Convert the RTree module query mechanism over to using a priority queue for
walking the RTree.
Performance optimization on nodeGetCell() in R-Tree.