http: expect 100 rework

Move all handling of HTTP's `Expect: 100-continue` feature into a client
reader. Add sending flag `KEEP_SEND_TIMED` that triggers transfer
sending on general events like a timer.

HTTP installs a `CURL_CR_PROTOCOL` reader when announcing `Expect:
100-continue`. That reader works as follows:

- on first invocation, records time, starts the `EXPIRE_100_TIMEOUT`
  timer, disables `KEEP_SEND`, enables `KEEP_SEND_TIMER` and returns 0,
  eos=FALSE like a paused upload.

- on subsequent invocation it checks if the timer has expired. If so, it
  enables `KEEP_SEND` and switches to passing through reads to the
  underlying readers.

Transfer handling's `readwrite()` will be invoked when a timer expires
(like `EXPIRE_100_TIMEOUT`) or when data from the server arrives. Seeing
`KEEP_SEND_TIMER`, it will try to upload more data, which triggers
reading from the client readers again. Which then may lead to a new
pausing or cause the upload to start.

Flags and timestamps connected to this have been moved from
`SingleRequest` into the reader's context.

Closes #13110

lib/sendf.c

@@ -579,6 +579,14 @@ CURLcode Curl_creader_def_unpause(struct Curl_easy *data,
   return CURLE_OK;
 }
 
+void Curl_creader_def_done(struct Curl_easy *data,
+                           struct Curl_creader *reader, int premature)
+{
+  (void)data;
+  (void)reader;
+  (void)premature;
+}
+
 struct cr_in_ctx {
   struct Curl_creader super;
   curl_read_callback read_cb;
@@ -840,6 +848,7 @@ static const struct Curl_crtype cr_in = {
   cr_in_resume_from,
   cr_in_rewind,
   Curl_creader_def_unpause,
+  Curl_creader_def_done,
   sizeof(struct cr_in_ctx)
 };
 
@@ -990,6 +999,7 @@ static const struct Curl_crtype cr_lc = {
   Curl_creader_def_resume_from,
   Curl_creader_def_rewind,
   Curl_creader_def_unpause,
+  Curl_creader_def_done,
   sizeof(struct cr_lc_ctx)
 };
 
@@ -1154,6 +1164,7 @@ static const struct Curl_crtype cr_null = {
   Curl_creader_def_resume_from,
   Curl_creader_def_rewind,
   Curl_creader_def_unpause,
+  Curl_creader_def_done,
   sizeof(struct Curl_creader)
 };
 
@@ -1250,6 +1261,7 @@ static const struct Curl_crtype cr_buf = {
   cr_buf_resume_from,
   Curl_creader_def_rewind,
   Curl_creader_def_unpause,
+  Curl_creader_def_done,
   sizeof(struct cr_buf_ctx)
 };
 
@@ -1307,3 +1319,24 @@ CURLcode Curl_creader_unpause(struct Curl_easy *data)
   }
   return result;
 }
+
+void Curl_creader_done(struct Curl_easy *data, int premature)
+{
+  struct Curl_creader *reader = data->req.reader_stack;
+  while(reader) {
+    reader->crt->done(data, reader, premature);
+    reader = reader->next;
+  }
+}
+
+struct Curl_creader *Curl_creader_get_by_type(struct Curl_easy *data,
+                                              const struct Curl_crtype *crt)
+{
+  struct Curl_creader *r;
+  for(r = data->req.reader_stack; r; r = r->next) {
+    if(r->crt == crt)
+      return r;
+  }
+  return NULL;
+
+}