Allow plugins to replace the WebSession implementation

Plugins can replace the existing implementation with the statement:
DynamicItem.bind(binder(), WebSession.class).to(...);
in a module designated as a "<Gerrit-HttpModule>" in the manifest.

Just the Cache implementation used for web sessions can be changed
by binding to a subclass of the now abstract CacheBasedWebSession
which supplies the Cache in the superclass constructor.

This is a step towards solving web session issues with multi-master.

Change-Id: I255661a62cfcbfe07646cb90005764061d618f3d

gerrit-httpd/src/main/java/com/google/gerrit/httpd/WebSessionManagerFactory.java

@@ -0,0 +1,22 @@
+// Copyright (C) 2013 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.httpd;
+
+import com.google.common.cache.Cache;
+import com.google.gerrit.httpd.WebSessionManager.Val;
+
+public interface WebSessionManagerFactory {
+  WebSessionManager create(Cache<String, Val> cache);
+}