use custom http client instead of the http DefaultClient (#823)

This commit replaces the default client from the http lib with a custom one.
The previously used default client doesn't come with a timeout. This could cause
unwanted locks.
That solution chosen here creates a http client in the base component dendrite
with a constant timeout of 30 seconds. If it should be necessary to overwrite
this, we could include the timeout in the dendrite configuration.
Here it would be a good idea to extend the type "Address" by a timeout and
create an http client for each service.

Closes #820

Signed-off-by: Benedikt Bongartz <benne@klimlive.de>

Co-authored-by: Kegsay <kegan@matrix.org>

appservice/api/query.go

@@ -20,6 +20,7 @@ package api
 import (
 	"context"
 	"database/sql"
+	"errors"
 	"net/http"
 
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
@@ -97,15 +98,15 @@ type httpAppServiceQueryAPI struct {
 
 // NewAppServiceQueryAPIHTTP creates a AppServiceQueryAPI implemented by talking
 // to a HTTP POST API.
-// If httpClient is nil then it uses http.DefaultClient
+// If httpClient is nil an error is returned
 func NewAppServiceQueryAPIHTTP(
 	appserviceURL string,
 	httpClient *http.Client,
-) AppServiceQueryAPI {
+) (AppServiceQueryAPI, error) {
 	if httpClient == nil {
-		httpClient = http.DefaultClient
+		return nil, errors.New("NewRoomserverAliasAPIHTTP: httpClient is <nil>")
 	}
-	return &httpAppServiceQueryAPI{appserviceURL, httpClient}
+	return &httpAppServiceQueryAPI{appserviceURL, httpClient}, nil
 }
 
 // RoomAliasExists implements AppServiceQueryAPI