RFR: 963: Skara mlbridge bot sometimes resends old messages

[Robin Westberg]

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

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Depends on: https://git.openjdk.java.net/skara/pull/1103

Commit messages:
 - Do not try to parse sent mails as a conversation as we just want the ID's

Changes: https://git.openjdk.java.net/skara/pull/1117/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1117&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-963
  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