fix: Synchronize API requests

Turned out that the Hetzner DNS API doesn't like multiple DNS records
created at the same time. And if there is now implicit dependency
between two resources, Terraform assumes it is safe to create more than
one resource at a time.

There is a bulk create endpoint, which might be the "right" way to
implement the resource. However, it would be neccessary to add a DNS
record group resource or something that groups a list of DNS records and
this make the CRUD operations of this resource more complicated.

I think having one resource per DNS record is good for now. Added a
Mutex to synchronize all API requests. I hope this is an acceptable
slow down. If there is demand for a DNS record set resources, let me
know. It shouldn't be hard to and probably a good first issue for a
contributer :-).

fixes #5

hetznerdns/api/client.go

@@ -9,6 +9,7 @@ import (
 	"log"
 	"net/http"
 	"strings"
+	"sync"
 )
 
 // UnauthorizedError represents the message of a HTTP 401 response
@@ -32,6 +33,7 @@ func defaultCreateHTTPClient() *http.Client {
 
 // Client for the Hetzner DNS API.
 type Client struct {
+	requestLock      sync.Mutex
 	apiToken         string
 	createHTTPClient createHTTPClient
 }
@@ -55,7 +57,16 @@ func (c *Client) doHTTPRequest(apiToken string, method string, url string, body
 		req.Header.Set("Content-Type", "application/json; charset=utf-8")
 	}
 
+	// This lock ensures that only one request is sent to Hetzber API
+	// at a time. See issue #5 for context.
+	// It seems that Terraform creates multiple resources simultanously
+	// and the API can't handle this right now.
+	c.requestLock.Lock()
+
 	resp, err := client.Do(req)
+
+	c.requestLock.Unlock()
+
 	if err != nil {
 		return nil, err
 	}
@@ -85,8 +96,6 @@ func parseUnprocessableEntityError(resp *http.Response) (*UnprocessableEntityErr
 	if err != nil {
 		return nil, fmt.Errorf("Error reading HTTP response body: %e", err)
 	}
-	log.Printf("[DEBUG] 422 Unprocessable Entity error response body: %s", string(body))
-
 	var unprocessableEntityError UnprocessableEntityError
 	err = parseJSON(body, &unprocessableEntityError)
 	if err != nil {