Add handler for notfound messages.
Add handler for notfound messages.

For now, these are ignored, but ultimately they should be used to
re-request the missing inventory from another peer.

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