Filter query by stopwords in xapian bridge

We now store stopwords in the database's metadata field.
Here we use those stopwords (if they exist) to create
a xapian stopper which is given to the query parser in order
to omit stopwords from its query.
[endlessm/eos-sdk#1983]
this'd just be `stopwords_data.map(stopper.add_word);`, since map takes a function as a parameter