Defer loading each DB Tasks class from AR DatabaseTasks

Because we don't need to load tasks for DBs that we don't use for the current app.
Also, these Tasks classes load AR::Base in their class level, and so immediately kick :active_record on_load hooks.
This used to happen when we were loading tasks, not when we run a task.

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -71,9 +71,9 @@ def register_task(pattern, task)
         @tasks[pattern] = task
       end
 
-      register_task(/mysql/,        ActiveRecord::Tasks::MySQLDatabaseTasks)
-      register_task(/postgresql/,   ActiveRecord::Tasks::PostgreSQLDatabaseTasks)
-      register_task(/sqlite/,       ActiveRecord::Tasks::SQLiteDatabaseTasks)
+      register_task(/mysql/,        'ActiveRecord::Tasks::MySQLDatabaseTasks')
+      register_task(/postgresql/,   'ActiveRecord::Tasks::PostgreSQLDatabaseTasks')
+      register_task(/sqlite/,       'ActiveRecord::Tasks::SQLiteDatabaseTasks')
 
       def db_dir
         @db_dir ||= Rails.application.config.paths["db"].first
@@ -288,11 +288,11 @@ def dump_schema_cache(conn, filename)
       private
 
         def class_for_adapter(adapter)
-          key = @tasks.keys.detect { |pattern| adapter[pattern] }
-          unless key
+          _key, task = @tasks.each_pair.detect { |pattern, _task| adapter[pattern] }
+          unless task
             raise DatabaseNotSupported, "Rake tasks not supported by '#{adapter}' adapter"
           end
-          @tasks[key]
+          task.is_a?(String) ? task.constantize : task
         end
 
         def each_current_configuration(environment)