Join with fill('null') should not deadlock

Nobody has claimed this yet.

I realize there is the following in https://docs.influxdata.com/kapacitor/v1.2/nodes/stats_node/:

> WARNING: It is not recommended to join the stats stream with the original data stream. Since they operate on different clocks you could potentially create a deadlock. This is a limitation of the current implementation and may be removed in the future.

However it wasn't clear to me whether that also applied for joins with the `.fill('null')` chain method or not. I also wasn't able to find the github issue that tracked the fix either way.

Below is a TICKscript and write script that can reproduce the issue somewhat reliably on Kapacitor v1.2.1 (git: 4628bda2e3fbcc43df694707ec3f640f213cb0dc):

TICKscript:
```
var from = stream
|from()
.measurement('test')

var stats = from
|stats(1s)

stats
|join(from)
.as('stats', 'from')
.tolerance(1s)
.fill('null')
|log()
```
Write script:
```
#!/usr/bin/env bash
nanos=$(date +%s)000000000
curl -i -XPOST 'http://localhost:9092/write?db=test&rp=default' --data-binary "test f1=0,f2=\"v2\""
sleep 1s
for i in `seq 1 10`;do
timestamp=$((nanos+i))
curl -i -XPOST 'http://localhost:9092/write?db=test&rp=default' --data-binary "test f1=$i,f2=\"v2\" $timestamp"
done
```
The output before it deadlocks:
```
[httpd] ::1 - - [20/Apr/2017:21:46:49 -0700] "POST /write?db=test&rp=default HTTP/1.1" 204 0 "-" "curl/7.51.0" 88421c89-264d-11e7-83e3-000000000000 91
[bug:log6] 2017/04/20 21:46:49 I! {"Name":"stats","Database":"","RetentionPolicy":"","Group":"","Dimensions":{"ByName":false,"TagNames":null},"Tags":{},"Fields":{"from.emitted":null,"stats.emitted":0},"Time":"2017-04-21T04:46:47Z"}

[bug:log6] 2017/04/20 21:46:50 I! {"Name":"stats","Database":"","RetentionPolicy":"","Group":"","Dimensions":{"ByName":false,"TagNames":null},"Tags":{},"Fields":{"from.emitted":null,"stats.emitted":0},"Time":"2017-04-21T04:46:48Z"}

[bug:log6] 2017/04/20 21:46:51 I! {"Name":"stats","Database":"","RetentionPolicy":"","Group":"","Dimensions":{"ByName":false,"TagNames":null},"Tags":{},"Fields":{"from.emitted":null,"stats.emitted":0},"Time":"2017-04-21T04:46:49Z"}

[bug:log6] 2017/04/20 21:46:52 I! {"Name":"stats","Database":"","RetentionPolicy":"","Group":"","Dimensions":{"ByName":false,"TagNames":null},"Tags":{},"Fields":{"from.f1":0,"from.f2":"v2","stats.emitted":0},"Time":"2017-04-21T04:46:50Z"}
```

Start by reproducing the deadlock with the provided TICKscript and bash write script on the Kapacitor v1.2.1 behavior described. Trace the join, stats, and fill('null') processing involved in the hang; done means the same input completes without deadlocking and produces the expected joined output.

Written by the indexing model from the issue text.