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.
