waddrmgr: update the Manager struct to remove functionality covered by ScopedManager
waddrmgr: update the Manager struct to remove functionality covered by ScopedManager

In this commit, we remove many of the methods in the Manager struct as
they’ve now be been replicated within a scoped format for each of the
ScopedKeyManagers.

A major change is that we’ll now actually store the master HD private
and public keys. This required as in order to create new scopes, we
need access to the master HD private key as hardened derivation is
required in accordance with BIP43.

The initial creation of the manager namespaces has also been extended
to create the namespaces and keys for the set of default key scopes.

Finally, a series of utility method has been added to allow callers to
create ScopedKeyManagers for arbitrary sets of scopes.

btcsuite/btcwallet,btcsuite/btcwallet