mempool: Improve tx input standard checks.
mempool: Improve tx input standard checks.

This changes the transaction input standardness checks as follows:

- Allow any script in a pay-to-script-hash transaction to be relayed and
  mined so long as it has no more than 15 signature operations
- Remove the obsolete checks which naively calculated the number of
  expected inputs in favor of the more accurate ScriptVerifyCleanStack
  and ScriptVerifySigPushOnly functionality of the script engine that was
  added after these checks.

stevenroose/btcd,jrick/dcrd,davecgh/btcd,Stash-Crypto/btcd,adiabat/btcd,ppcsuite/ppcd,jrick/btcd,BitGo/prova,jcmincke/dcrd,jcmincke/dcrd,dajohi/btcd,stevenroose/btcd,btcsuite/btcd,davecgh/btcd,ltcsuite/ltcd,BitGo/prova,Roasbeef/btcd,jongillham/btcd,aakselrod/btcd,decred/dcrd,ltcsuite/ltcd,Stash-Crypto/btcd,jrick/btcd,cqswxj2013/btcd,wallclockbuilder/btcd,jrick/dcrd,wallclockbuilder/btcd,dajohi/btcd,aakselrod/btcd,ppcsuite/ppcd,jongillham/btcd,dajohi/btcd,tb00/btcd,jcmincke/dcrd,Roasbeef/btcd,btcsuite/btcd,jrick/btcd,cqswxj2013/btcd,decred/dcrd,tb00/btcd,adiabat/btcd