Replication ignores since parameter if checkpoints are disabled

### Issue
When disabling replication checkpoints the `since` parameter is ignored. This is not indicated on the documentation and prevents me from managing my own replication checkpoints unless I also leave in redundant default checkpoints

### Info
- Environment: browser
- Platform: Chrome
- Adapter: idb
- Server: CouchDB

### Reproduce
```TypeScript
const localDb = new PouchDB('testdb');
const remoteDb = new PouchDB('https://sync.server.com:5984/testdb')
remoteDb.replicate.to(localDb, {
live: true,
retry: true,
since: '390525-g1AAAACheJzLYWBgYMpgTmEQTM4vTc5ISXIwNDLXMwBCwxyQVCJDUv3___-zMpiTGJg-fM8FirGbJVkYmFomY9ODx6Q8FiDJ0ACk_sMMZGZpAxtonmiZlGiO1cAsAI_hKv0',
checkpoint: false,
})
```
Check the network log and you will see it requests changes from sequence `0` rather than `390525`