CMake: added search for MySQL C++ Connector support

útil: hexify - generalized to work also for collections

ExtendedKey: support for loading and saving keys - also public ExtendedKeys can be saved as private DER/PEM encoded keys

BlockFilter/TransactionFilter/Peer/PeerManager: Fixed multiple request bug - now a get data request responding to a inv announcement is only sent to one peer, and retried after 45 seconds.

Chain: added support for cc(): the currency code, and Chain(int64) will nicely format the money!

Node: updatePeers command to update the peer list while online