fix(oohelperd): use throw-away HTTPClient, Dialer, Resolver (#833)

This diff modifies the implementation of oohelperd in the master branch
to always use throw-away HTTPClient, Dialer, and Resolver.

The rationale of this change is to ensure we're not hitting limits of the
HTTPClient regarding the max number of connections per host.

This issue is described at https://github.com/ooni/probe/issues/2182.

While there, it feels more correct to use throw-away Dialer and Resolver.

We have a different patch for the release/3.15 branch because of
netx-related refactorings: https://github.com/ooni/probe-cli/pull/832.

internal/cmd/oohelperd/internal/webconnectivity/webconnectivity.go

@@ -14,10 +14,10 @@ import (
 
 // Handler implements the Web Connectivity test helper HTTP API.
 type Handler struct {
-	Client            model.HTTPClient
-	Dialer            model.Dialer
 	MaxAcceptableBody int64
-	Resolver          model.Resolver
+	NewClient         func() model.HTTPClient
+	NewDialer         func() model.Dialer
+	NewResolver       func() model.Resolver
 }
 
 func (h Handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {