In this commit, we fix a slight bug in the existing implementation of
DeriveNextKey for btcwallet. Before this commit, we would only set the
public key, and not also the derivation path. It's important that we
also set the path information, as in the near future we'll be using the
KeyDescriptors returned from this method to create static channel back
ups. With these static backups, the key alone may be insufficient to
re-derive the private key as we may need to fallback to brute forcing in
order to re-derive the key as it's possible we add new key families in
the future.
This commit allows for secret keyrings to be initialized
with a specific coin type, which allows us to use
different derivation paths for bitcion and litecoin.
It also provide default constants for Bitcion and
Litecoin BIP 44 coin types.
