fix(agent): Retry agent heartbeats
fix(agent): Retry agent heartbeats

Attempt agent heartbeats three times with an exponential heartbeat.

Fix #116

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