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.

Fixes #2779 and #2780