Fix /become with new ActiveSession code

Fails because the externalId was null, but the column is
declared as NOT NULL.  Make it nullable, and also fix up
the token column to be more clear as session_cookie.

Change-Id: Ie0122d807976330878889c58ed86ceb0f88402a1
Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/ActiveSession.java b/gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/ActiveSession.java
index 608f952..f90add5 100644
--- a/gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/ActiveSession.java
+++ b/gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/ActiveSession.java
@@ -25,23 +25,23 @@
     private static final long serialVersionUID = 1L;
 
     @Column(id = 1, length = 60)
-    protected String token;
+    protected String sessionCookie;
 
     protected Key() {
     }
 
     public Key(final String token) {
-      this.token = token;
+      this.sessionCookie = token;
     }
 
     @Override
     public String get() {
-      return token;
+      return sessionCookie;
     }
 
     @Override
     protected void set(String newValue) {
-      token = newValue;
+      sessionCookie = newValue;
     }
   }
 
@@ -57,7 +57,7 @@
   @Column(id = 4)
   protected boolean persistentCookie;
 
-  @Column(id = 5)
+  @Column(id = 5, notNull = false)
   protected AccountExternalId.Key externalId;
 
   @Column(id = 6)