Allow pools used by Atomikos and Bitronix to be configured declaratively using the environment

[wilkinsona]

See this question on Stack Overflow. You have to do this at the moment:

@Bean
public XAConnectionFactoryWrapper xaConnectionFactoryWrapper() {
    return new XAConnectionFactoryWrapper() {
        @Override
        public ConnectionFactory wrapConnectionFactory(
                XAConnectionFactory connectionFactory) {
            PoolingConnectionFactoryBean pool = new PoolingConnectionFactoryBean();
            pool.setConnectionFactory(connectionFactory);
            pool.setMaxPoolSize(50);
            return pool;
        }

    };
}