Global Changes (_global_changes) database does not appear to work

[wwoods]

It looks like _global_changes doesn't work with this implementation. I understand this might be deliberate, but it should be documented somewhere, as it's a significant omitted feature.

If I've missed something required for _global_changes to work with pouchdb-server specifically, please let me know. I'm using code that works fine with a full CouchDB installation. Thanks!