Refactor keytime/metadata and wallet encryption bugfix

Refactor keytime:
* Key metadata is kept in a CWallet::mapKeyMetadata (std::map<CKeyId,CKeyMetadata>).
* When generating a new key, time is put in that map, and new key is written.
* AddKeyPubKey and AddCryptedKey do not take a creation time argument, but instead
  pull it from that map, if it exists there.

Bugfix:
* AddKeyPubKey and AddCryptedKey in CWallet didn't override the CKeyStore
  definition anymore. This is fixed, as they no longed need the nCreationTime
  argument now.

Also a few related other changes:
* Metadata can be overwritten.
* Only GenerateNewKey calls GetTime(), as it's the only place where we know for
  sure a key was not constructed earlier.
* When the nTimeFirstKey is known to be inaccurate, it is set to the value 1
  (instead of 0, which would mean unknown).
* Use CPubKey instead of std::vector<unsigned char> where possible.

src/walletdb.cpp

@@ -344,12 +344,14 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
         }
         else if (strType == "keymeta")
         {
-            vector<unsigned char> vchPubKey;
+            CPubKey vchPubKey;
             ssKey >> vchPubKey;
             CKeyMetadata keyMeta;
             ssValue >> keyMeta;
             wss.nKeyMeta++;
 
+            pwallet->LoadKeyMetadata(vchPubKey, keyMeta);
+
             // find earliest key creation time, as wallet birthday
             if (!pwallet->nTimeFirstKey ||
                 (keyMeta.nCreateTime < pwallet->nTimeFirstKey))
@@ -483,7 +485,7 @@ DBErrors CWalletDB::LoadWallet(CWallet* pwallet)
 
     // nTimeFirstKey is only reliable if all keys have metadata
     if ((wss.nKeys + wss.nCKeys) != wss.nKeyMeta)
-        pwallet->nTimeFirstKey = 0;
+        pwallet->nTimeFirstKey = 1; // 0 would be considered 'no value'
 
     BOOST_FOREACH(uint256 hash, wss.vWalletUpgrade)
         WriteTx(hash, pwallet->mapWallet[hash]);