Requests to `_changes` with `descending=true` ignore `since`

## Description

When passing `descending=true` to `_changes`, CouchDB completely ignores the `since` parameter and always resolves with results as if `since=now`. It even ignores a malformed seq.

## Steps to Reproduce

```
curl "http:////_changes?limit=1&descending=true&since=NOT_A_SEQ"
```

## Expected Behaviour

According to the documentation, `descending` means:
> Return the change results in descending sequence order (most recent change first). Default is false.

No other interactions are documented. I would expect it to respect the `since` parameter.

## Your Environment

* CouchDB version used: 3.0, 2.3.1, 2.2, 2.1, 1.7 **
* Operating system and version: Manjaro 19

** Couch 1.7 does return a badarg when passing a malformed `since` , but a correct `since` is ignored.

Start by reproducing the reported request with descending=true and since=NOT_A_SEQ, then trace the _changes request handling in CouchDB. Done means descending results respect a supplied since value and malformed sequences are handled consistently, with regression coverage for both cases.

Written by the indexing model from the issue text.