Wallet: Support disabling implicit Segwit operation

[luke-jr]

This optional feature (disabled by default) leaves Segwit wallet support mostly as-is, except:

1. It disables automatic implied Segwit recognition for keys not explicitly generated as Segwit. This means each Segwit address generated needs a new backup, same as previous versions.
2. Default address/change type is legacy.

OUTPUT_TYPE_DEFAULT is changed to an actual OutputType value, in expectation of reuse by #12119 as well as enabling the possibility of having the implicit segwit flag be set per-wallet once we have runtime wallet loading.

[TheBlueMatt]

I see little reason to do this. While we shouldn't make a habit of recognizing outputs to mutations of addresses we've given out, if someone sends you funds to a SegWit address, and you wish to refuse it, you're welcome to not honor the payment. Instead, we should be moving towards longer-term goals like splitting the HD chain across address types and recognizing payment only to addresses we've given out in a non-backup-breaking way.

[luke-jr]

Without this, there is currently no way to even detect they were sent to a Segwit mutation of an address.

[jnewbery]

Much discussion of this PR here: https://botbot.me/freenode/bitcoin-core-dev/2018-01-11/?msg=95652211&page=5

I'd support fixing the general case of address malleation. I don't think this is the correct approach, and don't think it should hold up v0.16, for the reasons enumerated in IRC. Concept NACK from me.

[luke-jr]

Rebased (which simplified it quite a bit).

[maflcko]

Closing for now. Let me know when you want to continue working on this.