Add atomic option in AbstractDB.{write,remove}

[tsirif]

Relative discussion.

We would like to support fetching multiple matched documents on a write (meant as an update) or remove, in an atomic way. Preferably using a single command, if the database framework supports it. Else, probably a locking mechanism should be devised and used (see #10).

A first approach,currently returning only one document and applying the update or remove on that, lies at tsirif/orion@ab159cf, on branch feature/db/atomic