Switch to using memcache as the session store

As a temporary measure any session which is not found in memcache
will be looked for in the database instead.

config/initializers/session_store.rb

@@ -1,8 +1,35 @@
 # Be sure to restart your server when you modify this file.
 
-if STATUS == :database_offline or STATUS == :database_readonly
-  OpenStreetMap::Application.config.session_store :cookie_store, :key => '_osm_session'
+if defined?(MEMCACHE_SERVERS)
+  unless STATUS == :database_offline or STATUS == :database_readonly
+    module Rack
+      module Session
+        class Memcache
+          def get_session(env, sid)
+            with_lock(env, [nil, {}]) do
+              unless sid and session = @pool.get(sid)
+                if sid and s = ActiveRecord::SessionStore::SqlBypass.find_by_session_id(sid)
+                  session = s.data
+                  s.destroy
+                else
+                  sid, session = generate_sid, {}
+                end
+                
+                unless /^STORED/ =~ @pool.add(sid, session)
+                  raise "Session collision on '#{sid.inspect}'"
+                end
+              end
+              [sid, session]
+            end
+          end
+        end
+      end
+    end
+  end
+
+  cache = MemCache.new(:namespace => "rails:session", :string_return_types => true)
+
+  OpenStreetMap::Application.config.session_store :mem_cache_store, :cache => cache, :key => "_osm_session"
 else
-  ActiveRecord::SessionStore.session_class = ActiveRecord::SessionStore::SqlBypass
-  OpenStreetMap::Application.config.session_store :active_record_store, :key => '_osm_session'
+  OpenStreetMap::Application.config.session_store :cookie_store, :key => '_osm_session'
 end