plug goroutine leak in control
plug goroutine leak in control

Fixes #2002.

weaveworks/scope,weaveworks/scope,alban/scope,kinvolk/scope,kinvolk/scope,paulbellamy/scope,alban/scope,paulbellamy/scope,alban/scope,weaveworks/scope,paulbellamy/scope,paulbellamy/scope,kinvolk/scope,weaveworks/scope,alban/scope,alban/scope,kinvolk/scope,weaveworks/scope,alban/scope,paulbellamy/scope,kinvolk/scope,paulbellamy/scope,kinvolk/scope,weaveworks/scope