KeychainTxOutIndex API inconsistent with respect to references (should be owned)

[LLFourn]

Methods often take a reference to a K or an owned K. They often return a reference or sometimes an owned K. I think we should always just used owned. Also I think we should owned ScriptBuf's rather than Script. The reason is they are both usually small and fine to copy:

• K is usually small and needs to be anyway if you derive lots of spks from a keychain (it is copied each time you do that into the underlying SpkTxOutIndex.
• script pubkeys are usually small unless you are doing stuff like DoSing the mempool/utxo set which we don't need to support.

It should be owned everywhere and this should be brought up to the Wallet API.

[nondiremanuel]

@rustaceanrob proposed to be assigned for this during the call (@notmandatory)

[rustaceanrob]

Yeah I will look at this