smarter network code, fixed various issues

- deciding on the stream at creation time is just silly - it should be done based on the identities (this part is TODO)
- changed NodeRegistry so it doesn't store nodes - this should help to connect faster
- inventory items shouldn't be advertised to nodes that are already aware of them (issue #13)
- objects shouldn't be requested more than once (issue #9)

demo/src/main/java/ch/dissem/bitmessage/demo/Application.java

@@ -43,11 +43,10 @@ public class Application {
         ctx = new BitmessageContext.Builder()
                 .addressRepo(new JdbcAddressRepository(jdbcConfig))
                 .inventory(new JdbcInventory(jdbcConfig))
-                .nodeRegistry(new JdbcNodeRegistry(jdbcConfig))
+                .nodeRegistry(new MemoryNodeRegistry())
                 .messageRepo(new JdbcMessageRepository(jdbcConfig))
                 .networkHandler(new NetworkNode())
                 .port(48444)
-                .streams(1)
                 .build();
 
         ctx.startup(new BitmessageContext.Listener() {