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

tests/http/testenv/mod_curltest/mod_curltest.c

@@ -425,7 +425,7 @@ static int curltest_put_handler(request_rec *r)
   apr_off_t rbody_len = 0;
   const char *s_rbody_len;
   const char *request_id = "none";
-  apr_time_t chunk_delay = 0;
+  apr_time_t read_delay = 0, chunk_delay = 0;
   apr_array_header_t *args = NULL;
   long l;
   int i;
@@ -450,6 +450,12 @@ static int curltest_put_handler(request_rec *r)
           request_id = val;
           continue;
         }
+        else if(!strcmp("read_delay", arg)) {
+          rv = duration_parse(&read_delay, val, "s");
+          if(APR_SUCCESS == rv) {
+            continue;
+          }
+        }
         else if(!strcmp("chunk_delay", arg)) {
           rv = duration_parse(&chunk_delay, val, "s");
           if(APR_SUCCESS == rv) {
@@ -478,6 +484,9 @@ static int curltest_put_handler(request_rec *r)
   ct = apr_table_get(r->headers_in, "content-type");
   ap_set_content_type(r, ct? ct : "text/plain");
 
+  if(read_delay) {
+    apr_sleep(read_delay);
+  }
   bb = apr_brigade_create(r->pool, c->bucket_alloc);
   /* copy any request body into the response */
   if((rv = ap_setup_client_block(r, REQUEST_CHUNKED_DECHUNK))) goto cleanup;