Split `NetworkGraph` message handling fns into unsigned and signed

Split `NetworkGraph` message handling fns into unsigned and signed

This takes the now-public `NetworkGraph` message handling functions
and splits them all into two methods - one which takes a required
Secp256k1 context and verifies signatures and one which takes only
the unsigned part of the message and does not take a Secp256k1
context.

This both clarifies the public API as well as simplifies it, all
without duplicating code.

Finally, this adds an assertion in the Router fuzzer to make sure
the constants used for message deserialization are correct.