Add AssigneeStatusUpdate

Keep details for assignee changes, so that it would be possible to query
for changes in the assignee field.

Assignee and pastAssignees are removed from ChangeNotesParser as they
are not needed.
Assignee and pastAssignee are also removed from cache as they can be
inferred.

Change-Id: I57cd4a5bd5f5886daee26f42a308f0dad1bd8b8b

java/com/google/gerrit/server/AssigneeStatusUpdate.java

@@ -0,0 +1,21 @@
+package com.google.gerrit.server;
+
+import com.google.auto.value.AutoValue;
+import com.google.gerrit.entities.Account;
+import java.sql.Timestamp;
+import java.util.Optional;
+
+/** Change to an assignee's status. */
+@AutoValue
+public abstract class AssigneeStatusUpdate {
+  public static AssigneeStatusUpdate create(
+      Timestamp ts, Account.Id updatedBy, Optional<Account.Id> currentAssignee) {
+    return new AutoValue_AssigneeStatusUpdate(ts, updatedBy, currentAssignee);
+  }
+
+  public abstract Timestamp date();
+
+  public abstract Account.Id updatedBy();
+
+  public abstract Optional<Account.Id> currentAssignee();
+}