eddie-communication: implement communication layer APIs for higher layers

The communication API is composed of:

• connect: set up the local CoAP node and connect it to the network
• disconnect: tear down the local CoAP node
• discover: get the list of all available resources in the network
• send_message: send a message to the specified resource (ip, port and resource uri), using the correct CoAP method, specifying both the query and the body of the message.
• receive_message: return the message with the corresponding token value. This message is the answer to the previous request made by the CoAP node.

communication/include/CoapClient.h

@@ -69,6 +69,14 @@ class ThreadSafeStorage {
 public:
     std::mutex m_;
 
+    std::list<Key> get_valid_keys() {
+        std::unique_lock<decltype(m_)> lock(m_);
+
+        std::list<Key> return_container;
+        for (auto key: s_) return_container.push_back(key);
+
+        return return_container;
+    }
     /**
      * Add a new key to the set of valid keys
      * @param k the key to add