mining: update GBT generation to include witness commitment
mining: update GBT generation to include witness commitment

This commit updates the block template generation logic to only include
witness transactions once the soft-fork has activated and to also
include the OP_RETURN witness commitment (with additional block weight
accounting).

Stash-Crypto/btcd,wallclockbuilder/btcd,ltcsuite/ltcd,tb00/btcd,davecgh/btcd,dajohi/btcd,ppcsuite/ppcd,dajohi/btcd,ltcsuite/ltcd,Stash-Crypto/btcd,btcsuite/btcd,aakselrod/btcd,Roasbeef/btcd,dajohi/btcd,Roasbeef/btcd,btcsuite/btcd,tb00/btcd,ppcsuite/ppcd,aakselrod/btcd,davecgh/btcd,wallclockbuilder/btcd