transfer: Curl_sendrecv() and event related improvements

- Renames Curl_readwrite() to Curl_sendrecv() to reflect that it
  is mainly about talking to the server, not reads or writes to the
  client. Add a `nowp` parameter since the single caller already
  has this.
- Curl_sendrecv() now runs all possible operations whenever it is
  called and either it had been polling sockets or the 'select_bits'
  are set.
  POLL_IN/POLL_OUT are not always directly related to send/recv
  operations. Filters like HTTP/2, QUIC or TLS may monitor reverse
  directions. If a transfer does not want to send (KEEP_SEND), it
  will not do so, as before. Same for receives.
- Curl_update_timer() now checks the absolute timestamp of an expiry
  and the last/new timeout to determine if the application needs
  to stop/start/restart its timer. This fixes edge cases where
  updates did not happen as they should have.
- improved --test-event curl_easy_perform() simulation to handle
  situations where no sockets are registered but a timeout is
  in place.
- fixed bug in events_socket() that complained about removing
  a socket that was unknown, when indeed it had removed the socket
  just before, only it was the last in the list
- fixed conncache's internal handle to carry the multi instance
  (where the cache has one) so that operations on the closure handle
  trigger event callbacks correctly.
- fixed conncache to not POLL_REMOVE a socket twice when a conneciton
  was closed.

Closes #14561

lib/cf-socket.c

@@ -404,6 +404,9 @@ CURLcode Curl_socket_open(struct Curl_easy *data,
 static int socket_close(struct Curl_easy *data, struct connectdata *conn,
                         int use_callback, curl_socket_t sock)
 {
+  if(CURL_SOCKET_BAD == sock)
+    return 0;
+
   if(use_callback && conn && conn->fclosesocket) {
     int rc;
     Curl_multi_closed(data, sock);