ocis driver: synctime based etag propagation

[butonic]

This pr changes the etag from a dedicated property to a calculated property, based on fileid and mtime.
It also introduces a treetime (tmtime), similar to eos synctime, which is used to indicate when the subtree last changed.

The traatime accounting is enabled for nodes which have the user.ocis.propagation extended attribute set to 1. We could change this to a more readable format, eg none, sync or async. Then we could make the propagation strategy configurable. Currently, propagation is in sync, which does require a lot of updates.

• verify file and folder updates correctly trigger updates
• verify using the sync client
• refactor xattr code
• add changelog

I also turned all magic strings for extended attributes into constants.

the logic is the same as in eos: the parent nodes tmtime is checked and updated if it is older than the mtime of the changed node, up to the root.

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.