TODO: "TCP Fast Open" is done, add monitor pool connections

  curl_easy_set_cb() - sets a callback PLUS its callback data

1.15 TCP Fast Open

 RFC 7413 defines how to include data already in the TCP SYN handshake to

 reduce latency.

1.15 Monitor connections in the connection pool

 libcurl's connection cache or pool holds a number of open connections for the

 purpose of possible subsequent connection reuse. It may contain a few up to a

 significant amount of connections. Currently, libcurl leaves all connections

 as they are and first when a connection is iterated over for matching or

 reuse purpose it is verified that it is still alive.

 Those connections may get closed by the server side for idleness or they may

 get a HTTP/2 ping from the peer to verify that they're still alive. By adding

 monitoring of the connections while in the pool, libcurl can detect dead

 connections (and close them) better and earlier, and it can handle HTTP/2

 pings to keep such ones alive even when not actively doing transfers on them.

1.16 Try to URL encode given URL