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
80a3b830cc