RFR: 1846: Optimize the method MailmanListReader::conversations

[Guoxiong Li]

Hi all, 

This patch caches the finally unchanged emails instead of the http response at the previous months to improve the performance. And it add the emails to the end of the list to avoid memory copy.

Thanks for the review.

Best Regards,
-- Guoxiong

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Commit messages:
 - SKARA-1846

Changes: https://git.openjdk.org/skara/pull/1487/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1487&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1846
  Stats: 43 lines in 1 file changed: 29 ins; 2 del; 12 mod
  Patch: https://git.openjdk.org/skara/pull/1487.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1487/head:pull/1487

PR: https://git.openjdk.org/skara/pull/1487