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.h

@@ -208,6 +208,8 @@ struct Curl_crtype {
                           struct Curl_creader *reader, curl_off_t offset);
   CURLcode (*rewind)(struct Curl_easy *data, struct Curl_creader *reader);
   CURLcode (*unpause)(struct Curl_easy *data, struct Curl_creader *reader);
+  void (*done)(struct Curl_easy *data,
+               struct Curl_creader *reader, int premature);
   size_t creader_size;  /* sizeof() allocated struct Curl_creader */
 };
 
@@ -256,6 +258,8 @@ CURLcode Curl_creader_def_rewind(struct Curl_easy *data,
                                  struct Curl_creader *reader);
 CURLcode Curl_creader_def_unpause(struct Curl_easy *data,
                                   struct Curl_creader *reader);
+void Curl_creader_def_done(struct Curl_easy *data,
+                           struct Curl_creader *reader, int premature);
 
 /**
  * Convenience method for calling `reader->do_read()` that
@@ -361,6 +365,19 @@ CURLcode Curl_creader_resume_from(struct Curl_easy *data, curl_off_t offset);
  */
 CURLcode Curl_creader_unpause(struct Curl_easy *data);
 
+/**
+ * Tell all client readers that they are done.
+ */
+void Curl_creader_done(struct Curl_easy *data, int premature);
+
+/**
+ * Look up an installed client reader on `data` by its type.
+ * @return first reader with that type or NULL
+ */
+struct Curl_creader *Curl_creader_get_by_type(struct Curl_easy *data,
+                                              const struct Curl_crtype *crt);
+
+
 /**
  * Set the client reader to provide 0 bytes, immediate EOS.
  */