Prevent database migration from consuming all the RAM on the server

Prevent large production databases from consuming all the RAM on the server
and causing the migration to take forever. With this patch, objects are read
in batches from the database. This removes the need to hold ruby
objects for all rows for a type in memory at once.

db/migrate/20100714111653_build_initial_journals_for_acts_as_journalized.rb

@@ -37,7 +37,7 @@ class BuildInitialJournalsForActsAsJournalized < ActiveRecord::Migration
         activity_type = p.activity_provider_options.keys.first
 
         # Create initial journals
-        p.find(:all).each do |o|
+        p.find_each(:batch_size => 100 ) do |o|
           # Using rescue and save! here because either the Journal or the
           # touched record could fail. This will catch either error and continue
           begin