Read mergeable bit from cache

Change callers on the client side to get this from ChangeInfo
(populated by ChangeJson) rather than Change.

Continue writing to the database, to facilitate online migration.

Change-Id: I7273e0253d357ff80aa3baf2b833419749b958ef

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

@@ -45,6 +45,7 @@ public class ChangeDetail {
   protected SubmitType submitType;
   protected SubmitTypeRecord submitTypeRecord;
   protected boolean canSubmit;
+  protected boolean mergeable;
   protected List<ChangeMessage> messages;
   protected PatchSet.Id currentPatchSetId;
   protected PatchSetDetail currentDetail;
@@ -274,4 +275,12 @@ public class ChangeDetail {
   public boolean canEdit() {
     return canEdit;
   }
+
+  public void setMergeable(boolean m) {
+    mergeable = m;
+  }
+
+  public boolean isMergeable() {
+    return mergeable;
+  }
 }