fix(agent): clear empty entries when dropping peers
fix(agent): clear empty entries when dropping peers

When a job no longer has any peers, it does not need to
be tracked. This will help keep the memory usage of the
fleet agent under control.

dongsupark/fleet,flandr/fleet,webwlsong/fleet,bviswana101/fleet,coreos/fleet,ecnahc515/fleet,coreos/fleet,mischief/fleet,improbable-io/fleet,bviswana101/fleet,WPMedia/fleet,thepwagner/fleet,mischief/fleet,jonboulle/fleet,thepwagner/fleet,giantswarm/fleet,wuqixuan/fleet,hhoover/fleet,WPMedia/fleet,dalbani/fleet,flandr/fleet,dongsupark/fleet,mwitkow/fleet,mwhooker/fleet,improbable-io/fleet,mwitkow-io/fleet,jonboulle/fleet,ecnahc515/fleet,endocode/fleet,jonboulle/fleet,esatterwhite/fleet,esatterwhite/fleet,bcwaldon/fleet,mookjp/fleet,dongsupark/fleet,bcwaldon/fleet,dalbani/fleet,hhoover/fleet,paveltiunov/fleet,mookjp/fleet,mwhooker/fleet,endocode/fleet,mwitkow/fleet,improbable-io/fleet,ecnahc515/fleet,paveltiunov/fleet,webwlsong/fleet,bcwaldon/fleet,wuqixuan/fleet,dalbani/fleet,coreos/fleet,octoblu/fleet,octoblu/fleet,mwitkow-io/fleet,giantswarm/fleet,giantswarm/fleet,endocode/fleet