concatenate_all_fields and source / concatenate_source usage

[lucabelluccini]

We should at least warn users of concurrent use of concatenate_all_fields and source / concatenate_source usage.

  fingerprint {
    source => ["foo", "bar"]
    concatenate_all_fields => true
    target => "fingerprint"
    }

We should at least warn concatenate_all_fields triggers source being ignored.

The ideal would be to do not start the pipeline at all, but it might be a breaking change.