return both address types in RPC calls

Currently the wallet returns the non-confidential version of addresses, leading to user confusion sometimes, and requires a `validateaddress` call to re-derive the confidential address. This is non-optimal.

We could simply switch the default to returning CT addresses(which would break API), or return both address types to maintain it.

First we should audit where this affects users most, and see if there's a principled way of displaying the data.

edit: Another thought: a startup argument that switches to confidential, if known. No API breakage, no extra verbosity.