Start migration to notedb implementation of PatchSetApprovals

Encapsulate the current state of the migration in a NotesMigration
class. For each database table (or broader group of functionality)
that we migrate to notedb, configure a boolean indicating whether that
data should be read from notes, defaulting to false.

Unlike reads, NotesMigration contains just a single boolean
indicating that data should be written. We don't attempt to write
just some types of data; as the migration continues we will just
rewrite history.

Since most existing reads of the PatchSetApprovals table have been
migrated to ApprovalsUtil, most implementation changes happen there.
There are a few other implementations scattered around, and some that
will require a bit more work (e.g. stamping normalized approvals at
submit time).

Change-Id: I5676267d4de607c385e8c9917a89333863b9c9e7

gerrit-common/src/main/java/com/google/gerrit/common/data/LabelType.java

@@ -34,13 +34,18 @@ public class LabelType {
   }
 
   public static String checkName(String name) {
-    if (name == null || name.isEmpty()) {
-      throw new IllegalArgumentException("Empty label name");
-    }
+    checkNameInternal(name);
     if ("SUBM".equals(name)) {
       throw new IllegalArgumentException(
           "Reserved label name \"" + name + "\"");
     }
+    return name;
+  }
+
+  public static String checkNameInternal(String name) {
+    if (name == null || name.isEmpty()) {
+      throw new IllegalArgumentException("Empty label name");
+    }
     for (int i = 0; i < name.length(); i++) {
       char c = name.charAt(i);
       if ((i == 0 && c == '-') ||