message: rename get_root_key to get_cached_key
  this is not the root key anymore
diff: @@ -103,7 +103,7 @@ impl Wallet {
  pub fn account(&self, account_index: u32) -> Result<Account> {
  let account = bip44::Account::new(account_index)?;
  - let account_key = self.get_root_key().derive(self.derivation_scheme, account.get_scheme_value()).public();
  + let account_key = self.get_cached_key().derive(self.derivation_scheme, account.get_scheme_value()).public();
  Ok(Account::new(account, account_key, self.derivation_scheme))
  }
  @@ -248,9 +248,7 @@ impl Wallet {
  /// retrieve the root extended private key from the wallet but pre
  /// derived for the purpose and coin type.
  - ///
  - /// TODO: this function is not meant to be public
  - fn get_root_key<'a>(&'a self) -> &'a hdwallet::XPrv {
  + fn get_cached_key<'a>(&'a self) -> &'a hdwallet::XPrv {
  &self.cached_root_key
  }
  @@ -258,7 +256,7 @@ impl Wallet {
  ///
  /// TODO: this function is not meant to be public
  pub fn get_xprv(&self, addressing: &Addressing) -> hdwallet::XPrv {
  - self.get_root_key()
  + self.get_cached_key()
  .derive(self.derivation_scheme, addressing.account.get_scheme_value())
  .derive(self.derivation_scheme, addressing.change)
  .derive(self.derivation_scheme, addressing.index.get_scheme_value())
  