Enhancement: Don't check for presence of files in the backend before writing

Before, all backend implementations were required to return an error if the
file that is to be written already exists in the backend. For most backends,
that means making a request (e.g. via HTTP) and returning an error when the
file already exists.

This is not accurate, the file could have been created between the HTTP request
testing for it, and when writing starts, so we've relaxed this requeriment,
which saves one additional HTTP request per newly added file.

https://github.com/restic/restic/pull/1623
