XDCR - accomodate SGW xattr change from "import" to "_mou"

From SGW team:

As part of our work to better support eventing-SGW co-existence, we're going to add some additional metadata to the _import xattr. I'm proposing that we change the name of this xattr to the more general name _mou (metadata-only update). I expect SGW to be the only service updating this document at the moment, but it could potentially be used in future by eventing in scenarios they are performing cluster-local, metadata-only updates.

Associated ticket is linked to this MB