Bugfix: store() should never throw a DuplicateKeyException.

It would be extremely rare, but if two nodes were trying to simultaneously
store() the same record with the same Key, one could fail with a DKE.

Attempt a second 'update' if the insert fails.

src/main/java/com/samskivert/depot/DepotRepository.java

@@ -553,9 +553,12 @@ public abstract class DepotRepository
         }
 
         final boolean[] created = new boolean[1];
+        try {
             _ctx.invoke(new CachingModifier<T>(record, key, key) {
                 @Override
-            protected int invoke (Connection conn, DatabaseLiaison liaison) throws SQLException {
+                protected int invoke (Connection conn, DatabaseLiaison liaison)
+                    throws SQLException
+                {
                     if (_key != null) {
                         // run the update
                         int mods = builder.prepare(conn).executeUpdate();
@@ -590,6 +593,27 @@ public abstract class DepotRepository
                     stats.noteModification(pClass);
                 }
             });
+
+        } catch (DuplicateKeyException dke) {
+            if (key == null) {
+                throw dke; // how would this even happen?
+            }
+            // this should be very rare: the insert failed. Retry one more update.
+            _ctx.invoke(new CachingModifier<T>(record, key, key) {
+                @Override
+                protected int invoke (Connection conn, DatabaseLiaison liaison)
+                    throws SQLException
+                {
+                    builder.newQuery(update);
+                    return builder.prepare(conn).executeUpdate();
+                }
+                @Override
+                public void updateStats (Stats stats) {
+                    stats.noteModification(pClass);
+                }
+            });
+        }
+
         return created[0];
     }