## Bug description

The new funnels time conversion endpoint sometimes returns bins with negative time values. This makes it difficult to reflect accurate time conversion funnels data on the frontend. @Twixes @neilkakkar 

<img width="379" alt="Screen Shot 2021-07-13 at 9 14 42 PM" src="https://user-images.githubusercontent.com/13460330/125560350-3ef2eb03-5fc5-4c89-a0ab-988c945d7d87.png">

Example CH localhost [link ](http://localhost:8000/insights?insight=FUNNELS&properties=%5B%5D&filter_test_accounts=false&interval=day&events=%5B%7B%22id%22%3A%22%24pageview%22%2C%22name%22%3A%22%24pageview%22%2C%22type%22%3A%22events%22%2C%22order%22%3A0%7D%2C%7B%22id%22%3A%22%24pageview%22%2C%22name%22%3A%22%24pageview%22%2C%22type%22%3A%22events%22%2C%22order%22%3A1%7D%2C%7B%22id%22%3A%22%24pageview%22%2C%22name%22%3A%22%24pageview%22%2C%22type%22%3A%22events%22%2C%22order%22%3A2%7D%5D&display=FunnelsTimeToConvert&actions=%5B%5D&new_entity=%5B%5D) to reproduce bug.

Request Payload

<img width="612" alt="Screen Shot 2021-07-13 at 9 19 06 PM" src="https://user-images.githubusercontent.com/13460330/125560592-9e2a0670-606c-485a-a102-7e085c72f225.png">


## Expected behavior

No negative time bin values. 

## How to reproduce

1. Navigate to new funnels time conversion page. Make sure to turn on this FF: `4535-funnel-bar-viz`
2. Add a few identical actions/events
3. Open up network requests to see the response to `api/funnels`

## Environment

- [X] PostHog Cloud
- [ ] self-hosted PostHog, version/commit: _please provide_

#### *Thank you* for your bug report – we love squashing them!

ee/clickhouse/queries/funnels/test/test_funnel_time_to_convert.py
_create_person(distinct_ids=["user a"], team=self.team)
        _create_person(distinct_ids=["user b"], team=self.team)
        _create_person(distinct_ids=["user c"], team=self.team)
        _create_person(distinct_ids=["user d"], team=self.team)
        _create_event(event="step one", distinct_id="user a", team=self.team, timestamp="2021-06-08 18:00:00")
        _create_event(event="step two", distinct_id="user a", team=self.team, timestamp="2021-06-08 19:00:00")
        
        _create_event(event="step three", distinct_id="user a", team=self.team, timestamp="2021-06-08 21:00:00")
        _create_event(event="step one", distinct_id="user b", team=self.team, timestamp="2021-06-09 13:00:00")
        _create_event(event="step two", distinct_id="user b", team=self.team, timestamp="2021-06-09 13:37:00")
        
        _create_event(event="blah", distinct_id="user b", team=self.team, timestamp="2021-06-09 13:38:00")
        _create_event(event="step three", distinct_id="user b", team=self.team, timestamp="2021-06-09 13:39:00")
        _create_event(event="step one", distinct_id="user c", team=self.team, timestamp="2021-06-11 07:00:00")
        _create_event(event="step two", distinct_id="user c", team=self.team, timestamp="2021-06-12 06:00:00")
        
        _create_event(event="step one", distinct_id="user d", team=self.team, timestamp="2021-06-11 07:00:00")
        _create_event(event="blah", distinct_id="user d", team=self.team, timestamp="2021-06-12 07:00:00")
        
        _create_event(event="step two", distinct_id="user d", team=self.team, timestamp="2021-06-12 09:00:00")
        filter = Filter(
            data={
                "insight": INSIGHT_FUNNELS,