Implement websocket communication with kj-http

Now that capnp/kj provides http and websocket functions, replace
the excellent websocketpp library with the kj functions. This
removes a dependency and allows for more consistent idiomatic
code. Thanks websocketpp, it was great to have you along!

This should enable parts of the refactor described in #49

src/server.h

@@ -20,6 +20,7 @@
 #define LAMINAR_SERVER_H_
 
 #include <kj/async-io.h>
+#include <kj/compat/http.h>
 #include <capnp/message.h>
 #include <capnp/capability.h>
 #include <functional>
@@ -50,21 +51,19 @@ public:
     void addWatchPath(const char* dpath);
 
 private:
-    kj::Promise<void> acceptHttpClient(kj::Own<kj::ConnectionReceiver>&& listener);
     kj::Promise<void> acceptRpcClient(kj::Own<kj::ConnectionReceiver>&& listener);
     kj::Promise<void> handleFdRead(kj::AsyncInputStream* stream, char* buffer, std::function<void(const char*,size_t)> cb);
 
     void taskFailed(kj::Exception&& exception) override;
 
 private:
-    struct WebsocketConnection;
-    class HttpImpl;
-
     int efd_quit;
     capnp::Capability::Client rpcInterface;
     LaminarInterface& laminarInterface;
-    kj::Own<HttpImpl> httpInterface;
     kj::AsyncIoContext ioContext;
+    kj::HttpHeaderTable headerTable;
+    kj::Own<kj::HttpService> httpService;
+    kj::Own<kj::HttpServer> httpServer;
     kj::Own<kj::TaskSet> listeners;
     kj::TaskSet childTasks;
     kj::TaskSet httpConnections;