Add GetKeyBirthTimes to wallet

Compute safe lower bounds on the birth times of all wallet keys. For
pool keys or keys with metadata, the actually stored birth time is
used. For all others, the birth times are inferred from the wallet
transactions.

src/walletdb.cpp

@@ -365,7 +365,16 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
         {
             int64 nIndex;
             ssKey >> nIndex;
+            CKeyPool keypool;
+            ssValue >> keypool;
             pwallet->setKeyPool.insert(nIndex);
+
+            // If no metadata exists yet, create a default with the pool key's
+            // creation time. Note that this may be overwritten by actually
+            // stored metadata for that key later, which is fine.
+            CKeyID keyid = keypool.vchPubKey.GetID();
+            if (pwallet->mapKeyMetadata.count(keyid) == 0)
+                pwallet->mapKeyMetadata[keyid] = CKeyMetadata(keypool.nTime);
         }
         else if (strType == "version")
         {