Make _doc_ids _changes filter fast-path limit configurable

For the builtin filter `_docs_ids`, the maximum number of documents for which we trigger the optimized code path is hardcoded to 100 here:

https://github.com/apache/couchdb/blob/master/src/couch/src/couch_changes.erl#L32-L34

It would be helpful if this number was configurable.