Since unspendable outputs can't be spent, there is no threshold at which it would be uneconomic to spend them.
This primarily targets transaction outputs with `OP_RETURN`.
---
Initially based on:
commit 9cf0ae26350033d43d5dd3c95054c0d1b1641eda
Author: zathras-crypto <zathrasc@gmail.com>
Date: Wed Mar 25 02:04:02 2015 -0700
Changes:
- cherry-picked on top of bitcoin:master
- added RPC test for fundrawtransaction
