The Features::new() method is nonsense and doesn't describe what
features were being set - we introduce an empty() and supported()
constructors instead.
This change was made in the flat features BOLT PR, as if a channel
requires some unknown feature bits we should still rumor it, we just
shouldn't route through it.
This merges local and global features into one struct, which is
parameterized by where it appers. The parameterization restricts
which queries can be made and which features can be set, in line
with the latest BOLT 9.
Closes#427.
