In this commit, we remove the unnecessary NewWitnessAddress RPC. We do
this as, at the moment, we only expose the wallet to generate witness
addresses. Now that the RPC has been removed, its REST endpoint has been
assigned to the NewAddress RPC, which should be the de facto way of
having the wallet generate any type of supported address.
Fixes#1986.
This commit allows users to sign messages with their node's private key
with the SignMessage interface. The signatures are zbase32 encoded for
human readability/paste-ability. Others users can verify that a message
was signed by another node in their channel database with the
VerifyMessage interface.
