validation: Use existing chainstate in ChainstateManager::ProcessNewBlockHeaders
validation: Use existing chainstate in ChainstateManager::ProcessNewBlockHeaders

[META] This commit should be followed up by removing the comments and
       assertions meant only to show that the change is correct.

pataquets/namecoin-core,domob1812/bitcoin,prusnak/bitcoin,practicalswift/bitcoin,pataquets/namecoin-core,n1bor/bitcoin,JeremyRubin/bitcoin,mruddy/bitcoin,rnicoll/bitcoin,achow101/bitcoin,Xekyo/bitcoin,MeshCollider/bitcoin,MarcoFalke/bitcoin,jlopp/statoshi,domob1812/namecore,Xekyo/bitcoin,apoelstra/bitcoin,domob1812/bitcoin,GroestlCoin/GroestlCoin,dscotese/bitcoin,pstratem/bitcoin,fanquake/bitcoin,Xekyo/bitcoin,bitcoin/bitcoin,Sjors/bitcoin,ElementsProject/elements,yenliangl/bitcoin,tecnovert/particl-core,bitcoin/bitcoin,MeshCollider/bitcoin,JeremyRubin/bitcoin,ElementsProject/elements,sstone/bitcoin,tecnovert/particl-core,ajtowns/bitcoin,mm-s/bitcoin,jambolo/bitcoin,jamesob/bitcoin,qtumproject/qtum,jlopp/statoshi,pstratem/bitcoin,rnicoll/bitcoin,practicalswift/bitcoin,namecoin/namecoin-core,pataquets/namecoin-core,qtumproject/qtum,lateminer/bitcoin,apoelstra/bitcoin,yenliangl/bitcoin,mm-s/bitcoin,ajtowns/bitcoin,GroestlCoin/bitcoin,practicalswift/bitcoin,ElementsProject/elements,MarcoFalke/bitcoin,GroestlCoin/GroestlCoin,bitcoinsSG/bitcoin,MeshCollider/bitcoin,kallewoof/bitcoin,anditto/bitcoin,practicalswift/bitcoin,andreaskern/bitcoin,yenliangl/bitcoin,n1bor/bitcoin,andreaskern/bitcoin,MeshCollider/bitcoin,bitcoinsSG/bitcoin,particl/particl-core,qtumproject/qtum,pstratem/bitcoin,apoelstra/bitcoin,jambolo/bitcoin,tecnovert/particl-core,kallewoof/bitcoin,rnicoll/bitcoin,bitcoinsSG/bitcoin,Sjors/bitcoin,JeremyRubin/bitcoin,Xekyo/bitcoin,bitcoin/bitcoin,n1bor/bitcoin,anditto/bitcoin,kallewoof/bitcoin,namecoin/namecore,achow101/bitcoin,lateminer/bitcoin,jamesob/bitcoin,yenliangl/bitcoin,anditto/bitcoin,domob1812/namecore,rnicoll/bitcoin,bitcoinknots/bitcoin,sstone/bitcoin,Xekyo/bitcoin,mruddy/bitcoin,yenliangl/bitcoin,qtumproject/qtum,AkioNak/bitcoin,dscotese/bitcoin,sipsorcery/bitcoin,namecoin/namecoin-core,Sjors/bitcoin,pataquets/namecoin-core,namecoin/namecoin-core,particl/particl-core,MeshCollider/bitcoin,GroestlCoin/GroestlCoin,fanquake/bitcoin,jlopp/statoshi,n1bor/bitcoin,sstone/bitcoin,tecnovert/particl-core,particl/particl-core,andreaskern/bitcoin,prusnak/bitcoin,fanquake/bitcoin,mruddy/bitcoin,MarcoFalke/bitcoin,achow101/bitcoin,JeremyRubin/bitcoin,bitcoin/bitcoin,anditto/bitcoin,mm-s/bitcoin,particl/particl-core,jamesob/bitcoin,AkioNak/bitcoin,instagibbs/bitcoin,mruddy/bitcoin,dscotese/bitcoin,domob1812/bitcoin,domob1812/bitcoin,namecoin/namecoin-core,namecoin/namecoin-core,ElementsProject/elements,jambolo/bitcoin,AkioNak/bitcoin,jambolo/bitcoin,sstone/bitcoin,kallewoof/bitcoin,instagibbs/bitcoin,mruddy/bitcoin,jlopp/statoshi,sipsorcery/bitcoin,ajtowns/bitcoin,prusnak/bitcoin,domob1812/bitcoin,pataquets/namecoin-core,MarcoFalke/bitcoin,AkioNak/bitcoin,n1bor/bitcoin,bitcoinknots/bitcoin,ElementsProject/elements,AkioNak/bitcoin,sipsorcery/bitcoin,GroestlCoin/GroestlCoin,pataquets/namecoin-core,sstone/bitcoin,Sjors/bitcoin,GroestlCoin/bitcoin,GroestlCoin/bitcoin,instagibbs/bitcoin,apoelstra/bitcoin,MeshCollider/bitcoin,kallewoof/bitcoin,achow101/bitcoin,JeremyRubin/bitcoin,mm-s/bitcoin,ajtowns/bitcoin,andreaskern/bitcoin,bitcoin/bitcoin,fujicoin/fujicoin,jlopp/statoshi,prusnak/bitcoin,namecoin/namecore,particl/particl-core,anditto/bitcoin,practicalswift/bitcoin,ajtowns/bitcoin,bitcoin/bitcoin,jlopp/statoshi,namecoin/namecore,JeremyRubin/bitcoin,GroestlCoin/bitcoin,MarcoFalke/bitcoin,namecoin/namecore,andreaskern/bitcoin,namecoin/namecore,domob1812/bitcoin,tecnovert/particl-core,jamesob/bitcoin,lateminer/bitcoin,rnicoll/bitcoin,qtumproject/qtum,lateminer/bitcoin,prusnak/bitcoin,GroestlCoin/GroestlCoin,fujicoin/fujicoin,domob1812/namecore,mruddy/bitcoin,GroestlCoin/bitcoin,GroestlCoin/bitcoin,Xekyo/bitcoin,mm-s/bitcoin,bitcoinsSG/bitcoin,domob1812/namecore,dscotese/bitcoin,achow101/bitcoin,lateminer/bitcoin,anditto/bitcoin,qtumproject/qtum,pstratem/bitcoin,sipsorcery/bitcoin,instagibbs/bitcoin,andreaskern/bitcoin,sipsorcery/bitcoin,dscotese/bitcoin,jamesob/bitcoin,apoelstra/bitcoin,instagibbs/bitcoin,pstratem/bitcoin,sipsorcery/bitcoin,GroestlCoin/GroestlCoin,ajtowns/bitcoin,yenliangl/bitcoin,fanquake/bitcoin,tecnovert/particl-core,fanquake/bitcoin,Sjors/bitcoin,instagibbs/bitcoin,qtumproject/qtum,bitcoinknots/bitcoin,mm-s/bitcoin,fujicoin/fujicoin,rnicoll/bitcoin,ElementsProject/elements,fujicoin/fujicoin,bitcoinknots/bitcoin,particl/particl-core,jamesob/bitcoin,prusnak/bitcoin,pstratem/bitcoin,apoelstra/bitcoin,bitcoinsSG/bitcoin,practicalswift/bitcoin,jambolo/bitcoin,fanquake/bitcoin,lateminer/bitcoin,sstone/bitcoin,bitcoinknots/bitcoin,dscotese/bitcoin,fujicoin/fujicoin,AkioNak/bitcoin,fujicoin/fujicoin,kallewoof/bitcoin,MarcoFalke/bitcoin,namecoin/namecore,domob1812/namecore,domob1812/namecore,n1bor/bitcoin,namecoin/namecoin-core,jambolo/bitcoin,bitcoinsSG/bitcoin,achow101/bitcoin