ratelimit: redesign

Description of how this works in `docs/internal/RATELIMITS.ms`.

Notable implementation changes:
- KEEP_SEND_PAUSE/KEEP_SEND_HOLD and KEEP_RECV_PAUSE/KEEP_RECV_HOLD
  no longer exist. Pausing is down via blocked the new rlimits.
- KEEP_SEND_TIMED no longer exists. Pausing "100-continue" transfers
  is done in the new `Curl_http_perform_pollset()` method.
- HTTP/2 rate limiting implemented via window updates. When
  transfer initiaiting connection has a ratelimit, adjust the
  initial window size
- HTTP/3 ngtcp2 rate limitin implemnented via ack updates
- HTTP/3 quiche does not seem to support this via its API
- the default progress-meter has been improved for accuracy
  in "current speed" results.

pytest speed tests have been improved.

Closes #19384

lib/progress.h

@@ -26,6 +26,7 @@
 
 #include "curlx/timeval.h"
 
+struct Curl_easy;
 
 typedef enum {
   TIMER_NONE,
@@ -50,15 +51,23 @@ void Curl_pgrsSetUploadSize(struct Curl_easy *data, curl_off_t size);
 
 void Curl_pgrsSetDownloadCounter(struct Curl_easy *data, curl_off_t size);
 void Curl_pgrsSetUploadCounter(struct Curl_easy *data, curl_off_t size);
-void Curl_ratelimit(struct Curl_easy *data, struct curltime now);
-int Curl_pgrsUpdate(struct Curl_easy *data);
-void Curl_pgrsUpdate_nometer(struct Curl_easy *data);
 
+/* perform progress update, invoking callbacks at intervals */
+CURLcode Curl_pgrsUpdate(struct Curl_easy *data);
+/* perform progress update, no callbacks invoked */
+void Curl_pgrsUpdate_nometer(struct Curl_easy *data);
+/* perform progress update with callbacks and speed checks */
+CURLcode Curl_pgrsCheck(struct Curl_easy *data);
+
+/* Inform progress/speedcheck about receive pausing */
+void Curl_pgrsRecvPause(struct Curl_easy *data, bool enable);
+
+/* Reset sizes and couners for up- and download. */
+void Curl_pgrsReset(struct Curl_easy *data);
+/* Reset sizes for up- and download. */
 void Curl_pgrsResetTransferSizes(struct Curl_easy *data);
+
 struct curltime Curl_pgrsTime(struct Curl_easy *data, timerid timer);
-timediff_t Curl_pgrsLimitWaitTime(struct pgrs_dir *d,
-                                  curl_off_t speed_limit,
-                                  struct curltime now);
 /**
  * Update progress timer with the elapsed time from its start to `timestamp`.
  * This allows updating timers later and is used by happy eyeballing, where
@@ -69,4 +78,9 @@ void Curl_pgrsTimeWas(struct Curl_easy *data, timerid timer,
 
 void Curl_pgrsEarlyData(struct Curl_easy *data, curl_off_t sent);
 
+#ifdef UNITTESTS
+UNITTEST CURLcode pgrs_speedcheck(struct Curl_easy *data,
+                                  struct curltime *pnow);
+#endif
+
 #endif /* HEADER_CURL_PROGRESS_H */