Resolve #180 - refactor pmd processors

- SourceCodeProcessor now consistently calls rule sets start / end
    if cache is not up to date
 - Both Mono and MultiThread Processors rely on PmdRunnable, just using
    different execution strategies
 - This also fixes https://sourceforge.net/p/pmd/bugs/1511/

pmd-test/src/main/java/net/sourceforge/pmd/testframework/RuleTst.java

@@ -216,9 +216,7 @@ public abstract class RuleTst {
             ctx.setIgnoreExceptions(false);
             RuleSet rules = new RuleSet();
             rules.addRule(rule);
-            rules.start(ctx);
             p.getSourceCodeProcessor().processSourceCode(new StringReader(code), new RuleSets(rules), ctx);
-            rules.end(ctx);
         } catch (Exception e) {
             throw new RuntimeException(e);
         }