Changes added to the same release note at the same time are lost, mixed up or refused

Fixed by https://github.com/xwiki-contrib/application-releasenotes/pull/82 (e5289ce):

• the page of a new entry is filled in a copy of the loaded page;
• the entries of a release note are created one at a time on a wiki instance;
• when a save fails because another cluster node created the page in the meantime, the next number is tried.

Checked on a Docker IT wiki (XWiki 17.10.0) with parallel REST creations: 8 at once × 15 rounds went from 38/120 created (21 overwritten) to 120/120, and 16 workers × 15 from 75/240 to 240/240, each change stored once.