Add barebones but working implementation of model preload (#209, #235)

Add barebones but working implementation of model preload

* add config test for Preload hook
* improve TestProxyManager_StartupHooks
* docs for new hook configuration
* add a .dev to .gitignore

proxy/discardWriter.go

@@ -0,0 +1,27 @@
+package proxy
+
+import "net/http"
+
+// Custom discard writer that implements http.ResponseWriter but just discards everything
+type DiscardWriter struct {
+	header http.Header
+	status int
+}
+
+func (w *DiscardWriter) Header() http.Header {
+	if w.header == nil {
+		w.header = make(http.Header)
+	}
+	return w.header
+}
+
+func (w *DiscardWriter) Write(data []byte) (int, error) {
+	return len(data), nil
+}
+
+func (w *DiscardWriter) WriteHeader(code int) {
+	w.status = code
+}
+
+// Satisfy the http.Flusher interface for streaming responses
+func (w *DiscardWriter) Flush() {}