waddrmgr: introduce concept of scopes for key managers
waddrmgr: introduce concept of scopes for key managers

In this commit, we introduce the concept of scopes for individual key
managers. Each scope will lock down a key manager to a particular
purpose and coin type within the BIP0043 hierarchy. Each scope will
also have a set address type schema. This schema will be consulted when
creating addresses for a particular scoped key manager.

Finally, we introduce 3 new default scopes:
   * BIP 44
   * BIP 84
   * BIP 49++ (BIP49 but uses p2wkh for change addresses)

Roasbeef/btcwallet,Roasbeef/btcwallet