Add benchmarks for readVarInt.
Add benchmarks for readVarInt.

This commit adds a few benchmarks for the readVarInt function.

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