Merge #16475: wallet: Enumerate walletdb keys

fa6f22b wallet: Rename CWalletKey to OldKey (MarcoFalke)
fa6dc7f wallet: Enumerate walletdb keys (MarcoFalke)

Pull request description:

  It is nice to see all the keys that exists in a single enum

  Also, rename CWalletKey to OldKey and update the outdated documentation

ACKs for top commit:
  laanwj:
    ACK fa6f22b, I'm a big fan of this kind of change as it prevents typos, which can happen with 'magic' strings in the code.
  promag:
    ACK fa6f22b. @jnewbery suggestions are great followups, I think this is good enough.
  meshcollider:
    utACK fa6f22b
  achow101:
    Code review ACK fa6f22b
  fanquake:
    ACK fa6f22b - I had a quick look over, definitely prefer this to strings floating around everywhere.

Tree-SHA512: 8ac3abd5a0d22dac1d77b8f97fe1e16c2608d650f3e9d6dd1df2fd5aeb35ef6643dfd4cd5c162404bb0100343c927d66df04dc695507ffc84a6c667e603acc54

Author: fanquake

src/wallet/wallet.cpp

@@ -4627,12 +4627,6 @@ CKeyPool::CKeyPool(const CPubKey& vchPubKeyIn, bool internalIn)
     m_pre_split = false;
 }
 
-CWalletKey::CWalletKey(int64_t nExpires)
-{
-    nTimeCreated = (nExpires ? GetTime() : 0);
-    nTimeExpires = nExpires;
-}
-
 void CMerkleTx::SetMerkleBranch(const uint256& block_hash, int posInBlock)
 {
     // Update the tx's hashBlock

src/wallet/wallet.h

@@ -676,23 +676,18 @@ class COutput
     }
 };
 
-/** Private key that includes an expiration date in case it never gets used. */
-class CWalletKey
-{
-public:
+/** Private key that was serialized by an old wallet (only used for deserialization) */
+struct OldKey {
     CPrivKey vchPrivKey;
-    int64_t nTimeCreated;
-    int64_t nTimeExpires;
-    std::string strComment;
-    // todo: add something to note what created it (user, getnewaddress, change)
-    //   maybe should have a map<string, string> property map
-
-    explicit CWalletKey(int64_t nExpires=0);
-
     ADD_SERIALIZE_METHODS;
 
     template <typename Stream, typename Operation>
     inline void SerializationOp(Stream& s, Operation ser_action) {
+        // no longer used by the wallet, thus dropped after deserialization:
+        int64_t nTimeCreated;
+        int64_t nTimeExpires;
+        std::string strComment;
+
         int nVersion = s.GetVersion();
         if (!(s.GetType() & SER_GETHASH))
             READWRITE(nVersion);