Refactor keytime/metadata and wallet encryption bugfix

Refactor keytime:
* Key metadata is kept in a CWallet::mapKeyMetaData (std::map<CKeyId,CKeyMeta)
* When generating a new key, time is put in that map, and new key is written
* AddKey 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.