Part of #880

Describe the PR

Refine #1446 to be more useful

In the java module: remove SideEffectingVisitor, and adds a JavaVisitor interface with the following signature:

public interface JavaVisitor<P, R> extends ... {
  R visit(JavaNode node, P param);
}

This visitor subsumes the existing JavaParserVisitor, which now extends JavaVisitor<Object, Object>, and also SideEffectingVisitor<T>, which can be written JavaVisitor<T, Void> now.

Doing this means we can use a single jjtAccept implementation per node, and it accepts both the new generic form, and the old JavaParserVisitor (which stays for compatibility).

This PR also deprecates JavaParserVisitor and JavaParserVisitorAdapter, because they're redundant and unsafe (mostly, it's unclear whether null, or the data parameter should be returned).

The same changes were done to the JSP module to check that the javacc wrapper is up to date & language-independent

In pmd-core, this adds

• an interface AstVisitor<P, R>, to be a supertype of language-specific visitor interfaces.
• a method <P, R> R acceptVisitor(AstVisitor,P) to the Node interface, which is the "language-independent jjtAccept". jjtAccept methods are deprecated on language-specific interfaces, so that only that single method is depended on.
• an annotation @DeprecatedUntil700 to further distinguish among deprecated APIs on the 7.0.x branch

Related issues

• [java] Use default method on generated visitor interfaces #1446
• [core] Make visitors generic #880

Ready?

Other languages need to have their visitor generified in order to implement AstVisitor, which I guess we can do in one PR per module, later

• Added unit tests for fixed bug/feature
• Passing all unit tests
• Complete build ./mvnw clean verify passes (checked automatically by travis)
• Added (in-code) documentation (if needed)