cfilters: send flush

Since data can be held in connection filter buffers when sending gives
EAGAIN, add methods to query this and perform flushing of those buffers.

The transfer loop will continue sending until all upload data is
processed and the connection is flushed.

- add `CF_QUERY_SEND_PENDING` to query filters
- add `CF_CTRL_DATA_SEND_FLUSH` to flush filters
- change `Curl_req_want_send()` to query the connection
  if it needs flushing
- use `Curl_req_want_send()` to determine the POLLOUT
  in the PERFORMING multi state
- implement flush handling in the HTTP/2 connection filter

Closes #14271

lib/multi.c

@@ -1101,7 +1101,7 @@ static int perform_getsock(struct Curl_easy *data, curl_socket_t *sock)
       sock[sockindex] = conn->sockfd;
     }
 
-    if(CURL_WANT_SEND(data)) {
+    if(Curl_req_want_send(data)) {
       if((conn->sockfd != conn->writesockfd) ||
          bitmap == GETSOCK_BLANK) {
         /* only if they are not the same socket and we have a readable