Update highest_index_seen_ from the nodes received.
Update highest_index_seen_ from the nodes received.

It appears etcd sometimes sends a slightly out of date index in the
X-Etcd-Index header? This causes double notifications, which isn't the
end of the world, but are easy to fix.

katjoyce/certificate-transparency,benlaurie/certificate-transparency,aeijdenberg/certificate-transparency,RJPercival/certificate-transparency,kyprizel/certificate-transparency,ksmaheshkumar/certificate-transparency,aeijdenberg/certificate-transparency,ksmaheshkumar/certificate-transparency,taknira/certificate-transparency,eranmes/certificate-transparency,benlaurie/certificate-transparency,rep/certificate-transparency,kyprizel/certificate-transparency,lexibrent/certificate-transparency,AlCutter/certificate-transparency,google/certificate-transparency,pphaneuf/certificate-transparency,Martin2112/certificate-transparency,rep/certificate-transparency,aeijdenberg/certificate-transparency,eranmes/certificate-transparency,plietar/certificate-transparency,google/certificate-transparency,AlCutter/certificate-transparency,AlCutter/certificate-transparency,kyprizel/certificate-transparency,rep/certificate-transparency,eranmes/certificate-transparency,plietar/certificate-transparency,Martin2112/certificate-transparency,Martin2112/certificate-transparency,phad/certificate-transparency,Martin2112/certificate-transparency,benlaurie/certificate-transparency,kyprizel/certificate-transparency,AlCutter/certificate-transparency,grandamp/certificate-transparency,taknira/certificate-transparency,rep/certificate-transparency,phad/certificate-transparency,phad/certificate-transparency,ksmaheshkumar/certificate-transparency,pphaneuf/certificate-transparency,plietar/certificate-transparency,eranmes/certificate-transparency,AlCutter/certificate-transparency,taknira/certificate-transparency,pphaneuf/certificate-transparency,Martin2112/certificate-transparency,taknira/certificate-transparency,ksmaheshkumar/certificate-transparency,benlaurie/certificate-transparency,aeijdenberg/certificate-transparency,lexibrent/certificate-transparency,plietar/certificate-transparency,google/certificate-transparency,katjoyce/certificate-transparency,RJPercival/certificate-transparency,katjoyce/certificate-transparency,katjoyce/certificate-transparency,RJPercival/certificate-transparency,google/certificate-transparency,kyprizel/certificate-transparency,taknira/certificate-transparency,ksmaheshkumar/certificate-transparency,pphaneuf/certificate-transparency,rep/certificate-transparency,plietar/certificate-transparency,grandamp/certificate-transparency,plietar/certificate-transparency,taknira/certificate-transparency,eranmes/certificate-transparency,katjoyce/certificate-transparency,aeijdenberg/certificate-transparency,rep/certificate-transparency,katjoyce/certificate-transparency,grandamp/certificate-transparency,benlaurie/certificate-transparency,eranmes/certificate-transparency,Martin2112/certificate-transparency,RJPercival/certificate-transparency,benlaurie/certificate-transparency,ksmaheshkumar/certificate-transparency,AlCutter/certificate-transparency,kyprizel/certificate-transparency,plietar/certificate-transparency,eranmes/certificate-transparency,katjoyce/certificate-transparency,aeijdenberg/certificate-transparency,kyprizel/certificate-transparency,AlCutter/certificate-transparency,phad/certificate-transparency,ksmaheshkumar/certificate-transparency,lexibrent/certificate-transparency,rep/certificate-transparency,lexibrent/certificate-transparency,taknira/certificate-transparency,Martin2112/certificate-transparency,grandamp/certificate-transparency,benlaurie/certificate-transparency,aeijdenberg/certificate-transparency