Add GetNewDestination to CWallet to fetch new destinations

Instead of having the same multiple lines of code everywhere
that new destinations are fetched, introduce GetNewDestination as
a member function of CWallet which does the key fetching, label
setting, script generation, and destination generation.

src/wallet/test/wallet_tests.cpp

@@ -466,8 +466,9 @@ BOOST_FIXTURE_TEST_CASE(wallet_disableprivkeys, TestChain100Setup)
     wallet->SetMinVersion(FEATURE_LATEST);
     wallet->SetWalletFlag(WALLET_FLAG_DISABLE_PRIVATE_KEYS);
     BOOST_CHECK(!wallet->TopUpKeyPool(1000));
-    CPubKey pubkey;
-    BOOST_CHECK(!wallet->GetKeyFromPool(pubkey, false));
+    CTxDestination dest;
+    std::string error;
+    BOOST_CHECK(!wallet->GetNewDestination(OutputType::BECH32, "", dest, error));
 }
 
 // Explicit calculation which is used to test the wallet constant