wallet: warn against accidental unsafe older() import

BIP 379 allows height and time locks that have no consensus meaning in BIP 68 / BIP 112.
This is used by some protocols like Lightning to encode extra data, but is unsafe when
used unintentionally. E.g. older(65536) is equivalent to older(1).

This commit emits a warning when importing such a descriptor.

It introduces a helper ForEachNode to traverse all miniscript nodes.

src/wallet/test/walletload_tests.cpp

@@ -35,6 +35,7 @@ public:
     std::optional<int64_t> MaxSatisfactionWeight(bool) const override { return {}; }
     std::optional<int64_t> MaxSatisfactionElems() const override { return {}; }
     void GetPubKeys(std::set<CPubKey>& pubkeys, std::set<CExtPubKey>& ext_pubs) const override {}
+    std::vector<std::string> Warnings() const override { return {}; }
 };
 
 BOOST_FIXTURE_TEST_CASE(wallet_load_descriptors, TestingSetup)