STAC: fix subtle parsing-mode bugs

Basically we want to avoid accidentally reparsing the output of
the discourse parser.

In --parsing mode, we don't know what the labels are because we
have presumably live data, so we should read files from the
unannotated stage (and the units stage for dialogue acts), and
filter on unannotated stage for features

A second issue is that dialogue acts were missing from live mode
features because unannotated files do not have authors, but the
dialogue act extracted ones do, so we have to relax the doc
match accordingly
STAC features : fix empty annotator in unannotated