core/merge: Update remote metadata of trashed docs

[taratatach]

Since trashing a document on the remote Cozy changes its CouchDB
revision, we need to store this updated metadata in our local PouchDB.
This makes future changes on the remote doc and detecting that the
trashed remote document is up-to-date possible.

We were updating the remote revision only when the doc had previously
been moved and that move wasn't synchronized yet.
We're now saving the complete updated remote metadata whenever merging
a document trashing (this includes the trashed attribute).

Please make sure the following boxes are checked:

• PR is not too big
• it improves UX & DX in some way
• it includes unit tests matching the implementation changes
• it includes scenarios matching a new behaviour or has been manually tested
• it includes relevant documentation