revisit memory fix for fastmultigather

[ctb]

this commit in #430 deallocates the duplicate memory used for against_collection here. Unfortunately, the RSS does briefly spike to 2x because of this. We should switch to using into_par_iter() per luiz. From slack:

luizirber you want into_par_iter and into_iter
13h
luizirber
par_iter and iter return references, into_* returns values
13h
luizirber

   pub fn into_par_iter(&self) -> impl IndexedParallelIterator<Item = (&Collection, Idx, &Record)> {
        // first create a Vec of all triples (Collection, Idx, Record)
        self
            .collections
            .into_par_iter() // CTB: are we loading things into memory here? No...
            .flat_map(|c| c.into_iter().map(move |(_idx, record)| (c, _idx, record)))
    }

13h
luizirber
or something close to that?