This is meant to handle a quirk in which key descriptors obtained
through walletrpc.DeriveKey don't result in the derived key being
persisted to the wallet's database, unlike with DeriveNextKey. Due to
this and some fallback logic in the wallet with regards to empty key
locators, if a request only specified the compressed public key, the
signature returned would be over a different key, namely the one derived
from (family=0, index=0).
With this commit we add the ability to create a shared DH key by using
a custom node private key instead of the node's identity private key.
If no key locator is specified the node's identity private key will be
used as a fallback.
To allow signing of messages with any key in the key chain
we add two new methods to the signer RPC. These behave differently
to the methods with the same name in the main RPC as described
in the documentation comment.
In this commit, we add the ComputeInputScript which will allow callers
to obtain witnesses for all outputs under control of the wallet. This
allows external scripting of things like coin join, etc.
