Replaced MessageCache by Cache<T>

This class will be used to implement the offline cache for both messages
and events.

src/main/java/envoy/client/net/Client.java

@@ -10,6 +10,7 @@ import java.util.logging.Logger;
 import javax.naming.TimeLimitExceededException;
 
 import envoy.client.Config;
+import envoy.client.data.Cache;
 import envoy.client.data.LocalDb;
 import envoy.client.util.EnvoyLog;
 import envoy.data.*;
@@ -61,7 +62,7 @@ public class Client implements Closeable {
 	 *                   failed for some other reason
 	 * @since Envoy v0.2-alpha
 	 */
-	public MessageCache onlineInit(LoginCredentials credentials, LocalDb localDb) throws Exception {
+	public Cache<Message> onlineInit(LoginCredentials credentials, LocalDb localDb) throws Exception {
 		// Establish TCP connection
 		logger.info(String.format("Attempting connection to server %s:%d...", config.getServer(), config.getPort()));
 		socket = new Socket(config.getServer(), config.getPort());
@@ -71,7 +72,7 @@ public class Client implements Closeable {
 		receiver = new Receiver(socket.getInputStream());
 
 		// Create cache for unread messages
-		final MessageCache cache = new MessageCache();
+		final Cache<Message> cache = new Cache<>();
 
 		// Register user creation processor, contact list processor and message cache
 		receiver.registerProcessor(User.class, sender -> { logger.info("Acquired user object " + sender); this.sender = sender; });