SolrDocManager: fix _id handling

The id field is converted differently on upsert (`doc.pop("_id")`, which is later handled by pysolr) and update (querying for `str(doc['_id'])`). This means that updates fail for some types of ID, such as UUIDs, that end up with a different representation (e.g. `4e61d3c7-23a9-4b29-8411-e23ab3791a6f` vs. `4e61d3c723a94b298411e23ab3791a6f`).

There is a similar occurrence in `_stream_search()` but I'm not sure if it needs to be fixed.