Determine "Validity" of an AST

[apmoriarty]

Expand upon the work done in #880 to validate the query tree structure.

• Multiple JexlNode rebuilding visitors are not preserving proper parentage #880 ensured that all nodes have a proper parent
• Address extra parens introduced by several visitors. ASTDelayedPredicate detects if a node is already delayed. #1165 attempted to remove extra parens from a query tree
• additional work to stop double wrapping nodes #1235 attempted to ensure that all reference expressions have a parent reference node

@jwomeara suggested we add a mechanism in which we could drop all visitors into a "verify" state, so that after a visitor is done executing we could immediately validate the resulting query tree based on a number of criteria. So far the criteria might include

1. Proper parentage
2. Minimal parens/reference expressions (simplifies logic in other visitors)
3. Ensure reference expressions have parent reference nodes (some visitors trigger on a reference node)
4. Ensure And/Or nodes do not have a single child node (see Add a method to JexlASTHelper which checks for AND/OR nodes with a single child #1233)

Additional ideas and comments are welcome.

[apmoriarty]

Expressions like ((F == 'v') || F2 == 'v2') should reduce to (F == 'v' || F2 == 'v2'). Or more formally "no single term child of an intersection or union should be wrapped".

[lbschanno]

Just want to add a comment that #603 is a bug that directly affects the work here. To summarize, retrieving the source nodes for a marker node may re-parent the source node and invalidate the original query tree.