RFR: 963: Skara mlbridge bot sometimes resends old messages [v3]

[Robin Westberg]

> Avoid parsing the sent mail archive into conversations as we're only interested in the IDs that we have already sent.

Robin Westberg has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains eight commits:

 - Merge remote-tracking branch 'upstream/master' into mlbridge_archive_parse
 - Do not try to parse sent mails as a conversation as we just want the ID's
 - Fix build error after merge
 - Merge remote-tracking branch 'upstream/master' into mbox_crosspost
 - Rename MailingList -> MailingListReader
 - Fix configuration parsing
 - Add support for treating a set of mboxes as a single source of conversations
 - Improve mbox parsing

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Changes: https://git.openjdk.java.net/skara/pull/1117/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1117&range=02
  Stats: 30 lines in 2 files changed: 13 ins; 13 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/1117.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1117/head:pull/1117

PR: https://git.openjdk.java.net/skara/pull/1117