RFR: 2070: Get a placeholder message spot for integration message [v3]

[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.

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  update

-------------

Changes:
  - all: https://git.openjdk.org/skara/pull/1619/files
  - new: https://git.openjdk.org/skara/pull/1619/files/9648a9d4..f9fcfe2b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1619&range=02
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1619&range=01-02

  Stats: 3 lines in 1 file changed: 1 ins; 1 del; 1 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