lib-http: Assign an ID for each HTTP request and log it in debug lines.

The ID stays the same when request is retried. Added a "Req" prefix so it's
easier to search for the IDs. Based on patch by Stephan Bosch.

src/lib-http/http-client-private.h

@@ -60,6 +60,7 @@ struct http_client_request {
 	pool_t pool;
 	unsigned int refcount;
 	const char *label;
+	unsigned int id;
 
 	struct http_client_request *prev, *next;
 
@@ -445,7 +446,7 @@ static inline const char *
 http_client_request_label(struct http_client_request *req)
 {
 	if (req->label == NULL) {
-		return t_strdup_printf("[%s %s%s]",
+		return t_strdup_printf("[Req%u: %s %s%s]", req->id,
 			req->method, http_url_create(&req->origin_url), req->target);
 	}
 	return req->label;

src/lib-http/http-client-request.c

@@ -63,6 +63,7 @@ static struct http_client_request *
 http_client_request_new(struct http_client *client, const char *method, 
 		    http_client_request_callback_t *callback, void *context)
 {
+	static unsigned int id_counter = 0;
 	pool_t pool;
 	struct http_client_request *req;
 
@@ -71,6 +72,7 @@ http_client_request_new(struct http_client *client, const char *method,
 	req->pool = pool;
 	req->refcount = 1;
 	req->client = client;
+	req->id = ++id_counter;
 	req->method = p_strdup(pool, method);
 	req->callback = callback;
 	req->context = context;
@@ -485,7 +487,7 @@ static void http_client_request_do_submit(struct http_client_request *req)
 	req->authority = p_strdup(req->pool, authority);
 
 	/* debug label */
-	req->label = p_strdup_printf(req->pool, "[%s %s]", req->method, target);
+	req->label = p_strdup_printf(req->pool, "[Req%u: %s %s]", req->id, req->method, target);
 
 	/* update request target */
 	if (req->connect_tunnel || have_proxy)