XWIKI-22782: Only save modified xobjects

[tmortagne]

Jira URL

https://jira.xwiki.org/browse/XWIKI-22782

Changes

Description

• added a flag to track if a BaseObject was modified
• made sure that any modification of something in that BaseObject (properties, direct metadata of that object) update this flag
• when saving the document skip any BaseObject which indicate that it was not modified
• had to do some Serializable cleanup (revapi was getting a bit lost). It's not even clear to me what BaseElement and its derivatives are Serializable in the first place, but that's another story…

Clarifications

The general idea is to try to reduce the useless requests during document save. This pull request is focusing in making sure we don't re-save xobject which are identical to what's in the database already.

The save side of things is the easy part, most of the changes in the pull request are about trying to make sure we properly track modifications made to objects.

Executed Tests

Counting on existing test to validate that at least there is no regression.

TODO: various unit tests to check the behavior of the new BaseObject (actually BaseElement, since BaseObject is not the only element with this concept) dirty flag.

Expected merging strategy

• Prefers squash: Yes
• Backport on branches: stable-16.10.x