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/request.h

@@ -81,8 +81,6 @@ struct SingleRequest {
   int httpcode;                 /* error code from the 'HTTP/1.? XXX' or
                                    'RTSP/1.? XXX' line */
   int keepon;
-  struct curltime start100;      /* time stamp to wait for the 100 code from */
-  enum expect100 exp100;        /* expect 100 continue state */
   enum upgrade101 upgr101;      /* 101 upgrade state */
 
   /* Client Writer stack, handles transfer- and content-encodings, protocol