Clarify that redeemScript is often optional

Rebased-From: 6265ecc
Ignore too-long redeemScripts while loading wallet

This avoids that long redeemScripts that were grandfathered in
prevent the wallet from loading.

Fixes #4313.

Rebased-From: 18116b0
Modified comment to make it clear that pull request refers to Bitcoin Core.
Merge pull request #676 from rnicoll/1.8.1-dev-redeem

Redeem script changes from Bitcoin Core 0.9.3