indexeddb adapter: Unable to add key to index 'seq'

### Issue
I'm using CouchDB remotely, PouchDB locally. I've been using the PouchDB idb adapter for a long time. I recently switched to the indexeddb adapter so I can use the purge function, but I'm running into a problem with the indexeddb adapter.

When my application initializes, it replicates N number of documents from CouchDB->PouchDB. I specify N via a configuration parameter. The initial replication is one-way and is successful regardless of the number of documents replicated. If N is <= 1000, after replication I can locally add documents and update documents to PouchDB without issue. However, if N is > 1000, after replication I get an error from the indexeddb adapter whenever I try to add or update a document locally. I'm using PouchDB 8.0. Here's the error:


### Info
- Environment: (browser)
- Platform: (Chrome)
- Adapter: (indexeddb)
- Server: (CouchDB)