[FW][FIX] session: avoid concurrent updates

[fw-bot]

Steps to reproduce (in odoo)

• Open a spreadsheet
• do something such that there's a least one revisions
• leave the spreadsheet

=> non-deterministic concurrent update because we save the thumbnail and we snapshot at the same time.

I'm able to reproduce more when my laptop power setting is on "performance" compared to "balanced"

With this commit leaveSession is now asynchronous and returns a promise which resolves after the snapshot was done.
It allows to await this promise to trigger other RPC updating the same record.

Task: 4080148

Description:

description of this task, what is implemented and why it is implemented that way.

Task: TASK_ID

review checklist

• feature is organized in plugin, or UI components
• support of duplicate sheet (deep copy)
• in model/core: ranges are Range object, and can be adapted (adaptRanges)
• in model/UI: ranges are strings (to show the user)
• undo-able commands (uses this.history.update)
• multiuser-able commands (has inverse commands and transformations where needed)
• new/updated/removed commands are documented
• exportable in excel
• translations (_t("qmsdf %s", abc))
• unit tested
• clean commented code
• track breaking changes
• doc is rebuild (npm run doc)
• status is correct in Odoo

Forward-Port-Of: #4911

[robodoo]

[fw-bot]

This PR targets saas-17.4 and is part of the forward-port chain. Further PRs will be created up to master.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port