Allow use of native id for MongoDB

We currently use a field "ecm:id" in MongoDB to store the document id. However MongoDB allows us to use its native "_id" field as long as it's unique, which fits our needs.

When doing this we avoid having an extra field store, and an extra index on it.

–
Use of MongoDB native _id field can be activated in the repository's configuration using:

    <repository name="default" label="label.default.repository">
      <server>${nuxeo.mongodb.server}</server>
      <dbname>${nuxeo.mongodb.dbname}</dbname>
      ...
      <nativeId>true</nativeId>
      ...
    </repository>