blockchain: optimize HaveBlock (#720)
blockchain: optimize HaveBlock (#720)

If a block is known to exist in the memory chain or database then
there is no need to check the orphan pool.
Roasbeef/btcd,Roasbeef/btcd,jrick/dcrd,dajohi/btcd,jrick/dcrd,jrick/btcd,adiabat/btcd,ltcsuite/ltcd,jrick/btcd,ppcsuite/ppcd,jcmincke/dcrd,jrick/btcd,adiabat/btcd,dajohi/btcd,BitGo/prova,jcmincke/dcrd,BitGo/prova,btcsuite/btcd,dajohi/btcd,wallclockbuilder/btcd,cqswxj2013/btcd,decred/dcrd,davecgh/btcd,wallclockbuilder/btcd,davecgh/btcd,ltcsuite/ltcd,tb00/btcd,ppcsuite/ppcd,tb00/btcd,decred/dcrd,btcsuite/btcd,jongillham/btcd,Stash-Crypto/btcd,jcmincke/dcrd,aakselrod/btcd,stevenroose/btcd,Stash-Crypto/btcd,jongillham/btcd,aakselrod/btcd,cqswxj2013/btcd,stevenroose/btcd