txscript: Optimize multi sig redeem script func.
txscript: Optimize multi sig redeem script func.

This converts the MultisigRedeemScriptFromScriptSig function to make use
of the new finalOpcodeData function instead of the far less efficient
parseScript thereby significantly optimizing the function.

It also deprecates the error return since it really does not make sense
given the preconditions of the function.

Finally, the comment is modified to explicitly call out the script
version semantics.

The following is a before and after comparison of analyzing a very large
script:

benchmark                       old ns/op    new ns/op    delta
------------------------------------------------------------------
BenchmarkMultisigRedeemScript   153623       1830         -98.81%

benchmark                       old allocs   new allocs   delta
------------------------------------------------------------------
BenchmarkMultisigRedeemScript   1              0          -100.00%

benchmark                       old bytes    new bytes    delta
------------------------------------------------------------------
BenchmarkMultisigRedeemScript   466944       0            -100.00%

jrick/dcrd,decred/dcrd,jrick/dcrd,decred/dcrd