RFR: Keep track of branch progress per notifier

[Robin Westberg]

Hi all,

Please review this change that makes the repository notifier keep track of the latest reported branch separately for
each kind of notifier. This allows other notifications to proceed even if a single notifier fails (such as when the
issue tracker is unavailable but email still works).

Best regards,
Robin

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Commit messages:
 - Keep track of branch progress per notifier

Changes: https://git.openjdk.java.net/skara/pull/509/files
 Webrev: https://webrevs.openjdk.java.net/skara/509/webrev.00
  Stats: 271 lines in 9 files changed: 201 ins; 11 del; 59 mod
  Patch: https://git.openjdk.java.net/skara/pull/509.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/509/head:pull/509

PR: https://git.openjdk.java.net/skara/pull/509