Hangup inside recording strems

Kapacitor version: 0.13.0;

My tick script is like below:

``` tick
stream
// Select just the cpu measurement from our example database.
|from()
.measurement('cpu')
.where(lambda: "usage_idle" < 90)
|alert()
.id('{{ index .Tags "host" }}: {{ index .Tags "path" }}')
.message('{{ .ID }} is {{ .Level }} value:{{ index .Fields "usage_idle" }}')
.info(lambda: "usage_idle" < 90)
.warn(lambda: "usage_idle" < 80)
.crit(lambda: "usage_idle" < 20)
```

It seems kapacitor records too many streams, So I'm decrease where expression to below:

``` tick
.where(lambda: "usage_idle" < 30)
```

then I need to redefine the task, but kapacitor hangs up, seems it will process the redefine request after all streams be processed.

Start by reproducing the hang with the provided stream task, first using the usage_idle < 90 filter and then redefining it with < 30. Trace how recording streams are processed during task redefinition. Done means a task can be redefined without waiting indefinitely for all recorded streams to finish.

Written by the indexing model from the issue text.