peer: Optimize merkle block handling.
peer: Optimize merkle block handling.

This commit updates the merkle block handling to for the latest changes
to the btcutil API and optimizes it along the way.

Previously, the code was inefficiently reloading the transactions for
the matched hashes from the database instead of simply pulling them from
the full block that was used to create the merkle block.

davecgh/btcd,toddfries/btcd,qchain/btcd,Stash-Crypto/btcd,jongillham/btcd,soapboxsys/ombfullnode,CryptocurrencyCabal/htcd,cqswxj2013/btcd,BitGo/prova,ltcsuite/ltcd,jongillham/btcd,cqswxj2013/btcd,adiabat/btcd,runeaune/btcd,dajohi/btcd,hectorj/btcd,aakselrod/btcd,toddfries/btcd,jcmincke/dcrd,wallclockbuilder/btcd,mathiasrw/btcd,jrick/btcd,hectorj/btcd,tb00/btcd,1d4Nf6/btcd,dan-da/btcd,BitGo/prova,dajohi/btcd,decred/dcrd,tb00/btcd,CryptocurrencyCabal/htcd,Stash-Crypto/btcd,jrick/dcrd,jcmincke/dcrd,mathiasrw/btcd,stevenroose/btcd,runeaune/btcd,decred/dcrd,Cleawing/btcd,jrick/btcd,aakselrod/btcd,Roasbeef/btcd,btcsuite/btcd,soapboxsys/ombfullnode,Cleawing/btcd,adiabat/btcd,dan-da/btcd,ppcsuite/ppcd,1d4Nf6/btcd,dajohi/btcd,davecgh/btcd,stevenroose/btcd,wallclockbuilder/btcd,Roasbeef/btcd,btcsuite/btcd,pkothbauer/btcd,pkothbauer/btcd,jrick/dcrd,qchain/btcd,ppcsuite/ppcd,jcmincke/dcrd,jrick/btcd,ltcsuite/ltcd