Continuous queries: wrong behavior for first values when using "RESAMPLE EVERY ... FOR"

__Steps to reproduce:__

1. `CREATE DATABASE mydb`

2. Create the continuous query
```
CREATE CONTINUOUS QUERY "cq" ON "mydb"
RESAMPLE EVERY 1m FOR 2m
BEGIN
SELECT last("value") AS "value"
INTO "mydb"."autogen"."mymeasurement"
FROM "mydb"."autogen"."mymeasurment"
GROUP BY time(1m), *
FILL(previous)
END
```

3. **During the same minute:**
```
INSERT mymeasurement,mytag='1' value=1
INSERT mymeasurement,mytag='2' value=2
```

4. `SELECT * from "mydb"."autogen"."mymeasurement"`
```
name: mymeasurement
time mytag value
---- ----- -----
1561461758977401270 '1' 1
1561461759496979918 '2' 2

```

5. Wait a bit (for the continuous query to execute) ... `SELECT * from "mydb"."autogen"."mymeasurement"`
```
name: mymeasurement
time mytag value
---- ----- -----
1561461660000000000 '2' 1
1561461720000000000 '1' 1
1561461720000000000 '2' 2
1561461758977401270 '1' 1
1561461759496979918 '2' 2
```

__Expected behavior:__
```
name: mymeasurement
time mytag value
---- ----- -----
1561461720000000000 '1' 1
1561461720000000000 '2' 2
1561461758977401270 '1' 1
1561461759496979918 '2' 2
```

__Actual behavior:__
```
name: mymeasurement
time mytag value
---- ----- -----
1561461660000000000 '2' 1
1561461720000000000 '1' 1
1561461720000000000 '2' 2
1561461758977401270 '1' 1
1561461759496979918 '2' 2
```

__Environment info:__

* System info: Linux 4.15.0-51-generic x86_64
* InfluxDB version: InfluxDB v1.7.6 (git: 1.7 01c8dd416270f424ab0c40f9291e269ac6921964)
* Other relevant environment details: Using docker **influxdb:1.7.6**

__Config:__
Everything default. As in the public docker image **influxdb:1.7.6** hosted at : https://hub.docker.com/_/influxdb

Start by reproducing the CREATE DATABASE, continuous query, INSERT, and SELECT sequence against InfluxDB 1.7.6 or the referenced Docker image. Compare the first resampled values with the expected output; done means the extra earlier bucket and incorrect first values no longer appear.

Written by the indexing model from the issue text.