I switched to using mlog for logging, as opposed to writing directly to
Stderr. This gives us control over log levels, as well as coordination
so that we don't have multiple go-routines writing to stderr at the same
time.
I had previously made the mistake of thinking that the Curve25519 key
which is generated for each host to use in nebula communication could
also be used for signing. This is not the case, Ed25519 is used for
signing and is different thant Curve25519.
Rather than figuring out how to convert the Curve25519 key into an
Ed25519 key, which there is no apparent support for in the standard
library, I opted to instead ship a separate key just for signing with
each host. Doing this required a bit of refactoring in order to keep all
the different keys straight and ensure all data which needs a signature
still has it.
