Write PID in temporary file first.  Then rename it as the real
destination.  It will avoid the issue that the external process may
read the empty PID file because of race condition.

SETTINGS timeout can be configurable using
--frontend-http2-settings-timeout and
--backend-http2-settings-timeout.

Since we only use it once, we don't have to use ev_timer_again, and
stop timer manually.

We don't have to check activeness of SETTINGS ACK timer since we only
send SETTINGS frame without ACK only once per session at the moment.

Also, we say connection succeeded only when we got SETTINGS ACK from
peer, rather than when we just connected to the peer in TCP or TLS.

The difference between them are former has const qualifier to the |in|
parameter, which is desirable since it is effectively read-only.