RFR: 2070: Get a placeholder message spot for integration message

[Zhao Song]

As Magnus said in the issue description, sometimes it's hard for users to find the merge ready comment. So he would like to see that this message is added as soon as possible, after the bridgekeeper has done its job. 

In this patch, on the very beginning of CheckWorkItem, skara bot would post a placeholder message to the PR and update it later.

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Commit messages:
 - SKARA-2070

Changes: https://git.openjdk.org/skara/pull/1619/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1619&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2070
  Stats: 174 lines in 13 files changed: 43 ins; 14 del; 117 mod
  Patch: https://git.openjdk.org/skara/pull/1619.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1619/head:pull/1619

PR: https://git.openjdk.org/skara/pull/1619