The method `MailmanListReader::conversations` caches the http response of the emails and uses the method `Mbox::splitMbox` to get the emails everytime. It is good to cache the emails when they would not be changed in the future.

And the method `MailmanListReader::conversations` uses the method `addAll(0, ...)` to add the emails to the beginning of the list. It is good to add the emails to the end of the list so that the number of memory copy can be reduced.

A sub-task to solve SKARA-1644.