Move additional headers fetching to own function
Move additional headers fetching to own function

Also moves the call to happen directly after validation of a headers message
(rather than mixed in with other state updates for the peer), and removes an
incorrect comment in favor of one that explains why headers sync must continue
from the last header a peer has sent.

lateminer/bitcoin,particl/particl-core,bitcoinsSG/bitcoin,Xekyo/bitcoin,Xekyo/bitcoin,namecoin/namecore,bitcoinsSG/bitcoin,particl/particl-core,sipsorcery/bitcoin,fujicoin/fujicoin,Xekyo/bitcoin,instagibbs/bitcoin,jamesob/bitcoin,fanquake/bitcoin,sipsorcery/bitcoin,particl/particl-core,namecoin/namecore,domob1812/bitcoin,bitcoin/bitcoin,jambolo/bitcoin,instagibbs/bitcoin,kallewoof/bitcoin,sipsorcery/bitcoin,fanquake/bitcoin,Xekyo/bitcoin,lateminer/bitcoin,jamesob/bitcoin,sipsorcery/bitcoin,domob1812/bitcoin,particl/particl-core,kallewoof/bitcoin,AkioNak/bitcoin,jambolo/bitcoin,bitcoinsSG/bitcoin,ajtowns/bitcoin,tecnovert/particl-core,sipsorcery/bitcoin,mruddy/bitcoin,mruddy/bitcoin,tecnovert/particl-core,fanquake/bitcoin,mruddy/bitcoin,namecoin/namecore,lateminer/bitcoin,jamesob/bitcoin,ajtowns/bitcoin,particl/particl-core,kallewoof/bitcoin,jamesob/bitcoin,AkioNak/bitcoin,namecoin/namecore,particl/particl-core,lateminer/bitcoin,namecoin/namecoin-core,tecnovert/particl-core,ajtowns/bitcoin,fanquake/bitcoin,namecoin/namecoin-core,bitcoin/bitcoin,AkioNak/bitcoin,sstone/bitcoin,jambolo/bitcoin,namecoin/namecoin-core,fujicoin/fujicoin,bitcoin/bitcoin,fujicoin/fujicoin,sstone/bitcoin,fujicoin/fujicoin,jamesob/bitcoin,bitcoinsSG/bitcoin,sstone/bitcoin,mruddy/bitcoin,kallewoof/bitcoin,mruddy/bitcoin,fujicoin/fujicoin,namecoin/namecore,bitcoin/bitcoin,bitcoinsSG/bitcoin,namecoin/namecoin-core,domob1812/bitcoin,ajtowns/bitcoin,jambolo/bitcoin,kallewoof/bitcoin,domob1812/bitcoin,AkioNak/bitcoin,tecnovert/particl-core,jambolo/bitcoin,fanquake/bitcoin,instagibbs/bitcoin,lateminer/bitcoin,AkioNak/bitcoin,tecnovert/particl-core,sstone/bitcoin,Xekyo/bitcoin,bitcoin/bitcoin,namecoin/namecore,namecoin/namecoin-core,namecoin/namecoin-core,tecnovert/particl-core,ajtowns/bitcoin,AkioNak/bitcoin,sipsorcery/bitcoin,ajtowns/bitcoin,fanquake/bitcoin,jamesob/bitcoin,sstone/bitcoin,bitcoinsSG/bitcoin,lateminer/bitcoin,instagibbs/bitcoin,fujicoin/fujicoin,instagibbs/bitcoin,sstone/bitcoin,domob1812/bitcoin,jambolo/bitcoin,bitcoin/bitcoin,instagibbs/bitcoin,kallewoof/bitcoin,Xekyo/bitcoin,domob1812/bitcoin,mruddy/bitcoin