Better debugging display of NXQL expressions

Currently when building NXQL expressions using predicates, printing their value for debugging can be misleading. For instance this may be displayed:

foo = 'f1' AND bar = 'b1' OR bar = 'b2'

where what we actually want is:

((foo = 'f1') AND ((bar = 'b1') OR (bar = 'b2')))

The parentheses make it clear what the operator priority is.