router: add HealthChecker interface
router: add HealthChecker interface

Related to #967.

cgvarela/tsuru,ggarnier/tsuru,kgrvamsi/tsuru,tsuru/tsuru,ArxdSilva/tsuru,guilhermebr/tsuru,kgrvamsi/tsuru,cgvarela/tsuru,cgvarela/tsuru,hcxiong/tsuru,gwmoura/tsuru,tuxmonteiro/tsuru,jblancett/tsuru,guilhermebr/tsuru,cezarsa/tsuru,Endika/tsuru,gwmoura/tsuru,gwmoura/tsuru,cezarsa/tsuru,hugoantunes/tsuru,ArxdSilva/tsuru,firebitsbr/tsuru,ggarnier/tsuru,4eek/tsuru,tuxmonteiro/tsuru,RichardKnop/tsuru,mderaldo/tsuru,guilhermebr/tsuru,kgrvamsi/tsuru,tuxmonteiro/tsuru,RichardKnop/tsuru,mderaldo/tsuru,hugoantunes/tsuru,gciotta/tsuru,tsuru/tsuru,Endika/tsuru,ArxdSilva/tsuru,ArxdSilva/tsuru,jblancett/tsuru,jblancett/tsuru,ggarnier/tsuru,firebitsbr/tsuru,mderaldo/tsuru,tsuru/tsuru,hcxiong/tsuru,RichardKnop/tsuru,4eek/tsuru,gciotta/tsuru,cezarsa/tsuru,Endika/tsuru,cezarsa/tsuru,tsuru/tsuru,hcxiong/tsuru,ggarnier/tsuru,4eek/tsuru,guilhermebr/tsuru,firebitsbr/tsuru,gciotta/tsuru