Inspector+LibCore+rpcdump: Rework the RPC stuff to be request/response

RPC clients now send JSON-encoded requests to the RPC server.
The connection also stays alive instead of disconnecting automatically
after the initial CObject graph dump.

JSON payloads are preceded by a single host-order encoded 32-bit int
containing the length of the payload.

So far, we have three RPC commands:

    - Identify
    - GetAllObjects
    - Disconnect

We'll be adding more of these as we go along. :^)

Userland/rpcdump.cpp

@@ -1,3 +1,5 @@
+#include <AK/JsonObject.h>
+#include <AK/JsonValue.h>
 #include <LibCore/CEventLoop.h>
 #include <LibCore/CLocalSocket.h>
 #include <stdio.h>
@@ -18,6 +20,13 @@ int main(int argc, char** argv)
 
     socket.on_connected = [&] {
         dbg() << "Connected to PID " << pid;
+
+        JsonObject request;
+        request.set("type", "GetAllObjects");
+        auto serialized = request.to_string();
+        i32 length = serialized.length();
+        socket.write((const u8*)&length, sizeof(length));
+        socket.write(serialized);
     };
 
     socket.on_ready_to_read = [&] {
@@ -32,6 +41,8 @@ int main(int argc, char** argv)
         for (int i = 0; i < data.size(); ++i)
             putchar(data[i]);
         printf("\n");
+
+        loop.quit(0);
     };
 
     auto success = socket.connect(CSocketAddress::local(String::format("/tmp/rpc.%d", pid)));