proxy: implement basic DhtProxyClient with the ability to get and put.

+ Add an abstract class to describe the Dht
+ Now, we can modify the Dht linked to a SecureDht. So we can choose
between the classic Dht and the Dht using a proxy.
+ Implement a basic client which can Get and Put values on the Dht using
the proxy.

Note: Update dhtnode to have the ability to use the DhtProxyClient.
Todo: Implement Listen, ping, connectivityChanged for the client.