LibIPC: Share most of the code between {Client,Server}Connection

This patch introduces IPC::Connection which becomes the new base class
of ClientConnection and ServerConnection. Most of the functionality
has been hoisted up to the base class since almost all of it is useful
on both sides.

This gives us the ability to send synchronous messages in both
directions, which is needed for the WebContent server process.
Unlike other servers, WebContent does not mind blocking on a response
from its client.

Services/ImageDecoder/ClientConnection.cpp

@@ -37,7 +37,7 @@ namespace ImageDecoder {
 static HashMap<int, RefPtr<ClientConnection>> s_connections;
 
 ClientConnection::ClientConnection(NonnullRefPtr<Core::LocalSocket> socket, int client_id)
-    : IPC::ClientConnection<ImageDecoderServerEndpoint>(*this, move(socket), client_id)
+    : IPC::ClientConnection<ImageDecoderClientEndpoint, ImageDecoderServerEndpoint>(*this, move(socket), client_id)
 {
     s_connections.set(client_id, *this);
 }

Services/ImageDecoder/ClientConnection.h

@@ -28,6 +28,7 @@
 
 #include <AK/HashMap.h>
 #include <ImageDecoder/Forward.h>
+#include <ImageDecoder/ImageDecoderClientEndpoint.h>
 #include <ImageDecoder/ImageDecoderServerEndpoint.h>
 #include <LibIPC/ClientConnection.h>
 #include <LibWeb/Forward.h>
@@ -35,7 +36,7 @@
 namespace ImageDecoder {
 
 class ClientConnection final
-    : public IPC::ClientConnection<ImageDecoderServerEndpoint>
+    : public IPC::ClientConnection<ImageDecoderClientEndpoint, ImageDecoderServerEndpoint>
     , public ImageDecoderServerEndpoint {
     C_OBJECT(ClientConnection);