Detect chain reorgs that happen during a rescan.
Detect chain reorgs that happen during a rescan.

While here, switch rescan method to use hashes, not heights, to refer
to starting and ending blocks.

Closes #151.

ok @davecgh

danoctavian/btcd,jimmysong/btcd,wallclockbuilder/btcd,pkothbauer/btcd,Evil-Coin/Evil,Dirbaio/btcd,cqswxj2013/btcd,decred/dcrd,DanielKrawisz/btcd,dan-da/btcd,toddfries/btcd,tb00/btcd,1d4Nf6/btcd,runeaune/btcd,Stash-Crypto/btcd,dan-da/btcd,FactomProject/btcd,davecgh/btcd,qchain/btcd,ltcsuite/ltcd,jrick/dcrd,tb00/btcd,decred/dcrd,wallclockbuilder/btcd,glv2/ppcd,dajohi/btcd,qchain/btcd,jrick/dcrd,jrick/btcd,sysbot/btcd,adiabat/btcd,btcsuitereleases/btcd,Dirbaio/btcd,stevenroose/btcd,Evil-Coin/Evil,FactomProject/btcd,jrick/btcd,jcmincke/dcrd,Cleawing/btcd,jongillham/btcd,ppcsuite/ppcd,ppcsuite/ppcd-old,CryptocurrencyCabal/htcd,btcsuite/btcd,ppcsuite/ppcd-old,CryptocurrencyCabal/htcd,dajohi/btcd,sysbot/btcd,runeaune/btcd,cqswxj2013/btcd,danoctavian/btcd,soapboxsys/ombfullnode,Stash-Crypto/btcd,cjepson/btcd,1d4Nf6/btcd,Roasbeef/btcd,aakselrod/btcd,Cleawing/btcd,jimmysong/btcd,jrick/btcd,Roasbeef/btcd,pkothbauer/btcd,btcsuite/btcd,jcmincke/dcrd,toddfries/btcd,jongillham/btcd,DanielKrawisz/btcd,jcmincke/dcrd,ltcsuite/ltcd,stevenroose/btcd,mathiasrw/btcd,davecgh/btcd,hectorj/btcd,btcsuitereleases/btcd,hectorj/btcd,cjepson/btcd,BitGo/prova,aakselrod/btcd,Dirbaio/btcd,dajohi/btcd,ppcsuite/ppcd,mathiasrw/btcd,soapboxsys/ombfullnode,adiabat/btcd,glv2/ppcd,BitGo/prova