Make sure idbnext performs as well or better than the existing adapter

With IDBNext's (`pouchdb-adapter-indexeddb`) major features complete, merged and ready to be pushed as a module, we should go through all of the perf tests and make sure it's as good or better than the current adapter in all cases.

From memory there was definitely an issue with saving attachments, but there are performance tests we can use as a reference to make sure we're covering everything.