Refactor CommandHandler interface

Replaces the messy CommandHandler interface with
a more separated design of

  BatchTransactionApplier
  TransactionApplier
  CommandVisitor

community/kernel/src/main/java/org/neo4j/graphdb/index/IndexImplementation.java

@@ -24,7 +24,7 @@
 import java.util.Map;
 
 import org.neo4j.graphdb.ResourceIterator;
-import org.neo4j.kernel.impl.transaction.command.CommandHandler;
+import org.neo4j.kernel.impl.api.CommandVisitor;
 import org.neo4j.kernel.lifecycle.Lifecycle;
 
 /**
@@ -57,7 +57,7 @@ public interface IndexImplementation extends Lifecycle
      * @param recovery indicate recovery
      * @return an index applier that will get notifications about commands to apply.
      */
-    CommandHandler newApplier( boolean recovery );
+    CommandVisitor newApplier( boolean recovery );
 
     /**
      * Fills in default configuration parameters for indexes provided from this

community/kernel/src/main/java/org/neo4j/kernel/impl/api/BatchTransactionApplier.java

@@ -0,0 +1,106 @@
+/*
+ * Copyright (c) 2002-2015 "Neo Technology,"
+ * Network Engine for Objects in Lund AB [http://neotechnology.com]
+ *
+ * This file is part of Neo4j.
+ *
+ * Neo4j is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.neo4j.kernel.impl.api;
+
+import java.io.IOException;
+import java.util.ArrayList;
+
+import org.neo4j.kernel.impl.locking.LockGroup;
+
+/**
+ * Responsible for dealing with batches of transactions. See also {@link TransactionApplier}
+ *
+ * Typical usage looks like:
+ * <pre>
+ * try ( BatchTransactionApplier batchApplier = getBatchApplier() )
+ * {
+ *     TransactionToApply tx = batch;
+ *     while ( tx != null )
+ *     {
+ *         try ( LockGroup locks = new LockGroup() )
+ *         {
+ *             ensureValidatedIndexUpdates( tx );
+ *             try ( TransactionApplier txApplier = batchApplier.startTx( tx, locks ) )
+ *             {
+ *                 tx.transactionRepresentation().accept( txApplier );
+ *             }
+ *         }
+ *         catch ( Throwable cause )
+ *         {
+ *             databaseHealth.panic( cause );
+ *             throw cause;
+ *         }
+ *         tx = tx.next();
+ *     }
+ * }
+ * </pre>
+ */
+public interface BatchTransactionApplier extends AutoCloseable
+{
+    // Once we don't have to validate index updates anymore, we can change this to simply be the transactionId
+    TransactionApplier startTx( TransactionToApply transaction ) throws IOException;
+    // If transactions need to share lock group
+    TransactionApplier startTx( TransactionToApply transaction, LockGroup lockGroup ) throws IOException;
+
+    class BatchTransactionApplierFacade implements BatchTransactionApplier
+    {
+
+        private final BatchTransactionApplier[] appliers;
+
+        public BatchTransactionApplierFacade( BatchTransactionApplier... appliers )
+        {
+            this.appliers = appliers;
+        }
+
+        @Override
+        public TransactionApplier startTx( TransactionToApply transaction ) throws IOException
+        {
+            ArrayList<TransactionApplier> txAppliers = new ArrayList<>();
+            for ( BatchTransactionApplier applier : appliers )
+            {
+                txAppliers.add( applier.startTx( transaction ) );
+            }
+            return new TransactionApplier.TransactionApplierFacade(
+                    txAppliers.toArray( new TransactionApplier[appliers.length] ) );
+        }
+
+        @Override
+        public TransactionApplier startTx( TransactionToApply transaction, LockGroup lockGroup ) throws IOException
+        {
+            ArrayList<TransactionApplier> txAppliers = new ArrayList<>();
+            for ( BatchTransactionApplier applier : appliers )
+            {
+                txAppliers.add( applier.startTx( transaction, lockGroup ) );
+            }
+            return new TransactionApplier.TransactionApplierFacade(
+                    txAppliers.toArray( new TransactionApplier[appliers.length] ) );
+        }
+
+        @Override
+        public void close() throws Exception
+        {
+            // Not sure why it is necessary to close them in reverse order
+            for ( int i = appliers.length; i-- > 0; )
+            {
+                appliers[i].close();
+            }
+        }
+    }
+}