ProjectConfig sections refactoring

Add a new class RefConfigSection to be superclass for some
sections in project.config file.
This is a preparation to include "Merge Strategy per branch"
feature which will add another section to project.config file.

Change-Id: I559aad8325358823471665b24186470a76aaf967

gerrit-server/src/main/java/com/google/gerrit/server/project/SectionMatcher.java

@@ -33,7 +33,7 @@ import java.util.regex.Pattern;
 abstract class SectionMatcher {
   static SectionMatcher wrap(AccessSection section) {
     String ref = section.getName();
-    if (AccessSection.isAccessSection(ref)) {
+    if (AccessSection.isValid(ref)) {
       return wrap(ref, section);
     } else {
       return null;