Task concurrency not being respected

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

1. Create a task that takes longer than its "every" property
2. Set the concurrency to 1
```
option task = {name: "Downsample raw data", every: 10s, offset: 10s, concurrency: 1}
```

__Expected behavior:__
I'd expect only one task to be started at a time.

__Actual behavior:__
Multiple tasks are started and running at the same time. Concurrency option is not being respected.

__Environment info:__

* System info: Windows 10 Enterprise LTSC 2019
* InfluxDB version:
InfluxDB v2.3.0+SNAPSHOT.090f681737
Server: [090f681](https://github.com/influxdata/influxdb/tree/090f681737)
Frontend: [a2bd1f3](https://github.com/influxdata/ui/tree/a2bd1f3)

**Update**
I've tried creating a task via Influx CLI also. Same thing.

Reproduce the task defined with every: 10s, offset: 10s, and concurrency: 1, using either the task configuration or the Influx CLI. Compare the running task instances with the expected single active task, then trace the task scheduler behavior for the reported InfluxDB v2.3.0 snapshot and add coverage showing that overlapping runs are prevented.

Written by the indexing model from the issue text.