Disable cashaddress on BCH node

Here is a quick little note for those who may be upgrading Bitcoin ABC to 0.17 for the hard fork.

About a week ago we built a container to comply with the Bitcoin Cash hardfork but there was an issue deploying.

All of the addresses returned by the node are in the new Bitcoin Cash address format.

I shot an email to the Bitcoin ABC team and thankfully they are very responsive. They let me know that if you add a setting usecashaddr=0 the node will revert to using the old address format.

This flag isn't really documented so I'm adding it here to hopefully save some other folks from having issues.