nghttpx: Add frontend-keep-alive-timeout option

on_read may fail, but we failed to check its return value most of the
places.  This is because failure means deletion of ClientHandler, but
because of architecture, we cannot delete it.  Feeding read event is
better since we can move call on_read from libev callback.  We can
delete ClientHandler form there.

Because of bidirectional nature of TCP, we may fail write(2), but have
still pending read in TCP buffer, which may contain response body.  To
forward them, we should keep reading until get EOF from backend.

To avoid stalling HTTP/1 upload when request buffer is full, and we
have received complete response from backend, drop connection in that
case.

To debug multi threaded configuration easier, we added current PID and
thread ID to error log.  Previously, we didn't add date and time if
log level is NOTICE.  In this change, we always write date and time
regardless of log level.

Use pkg-config to detect libxml2

examples: Delete tiny-nghttpd

nghttpd does the better job in this area.

Fix bug that fetch-ocsp-response does not work with OpenSSL 1.1.0

The syntax of openssl ocsp -header option has been changed in OpenSSL
1.1.0.  And it now does not require -header option anymore.  It looks
like that it is workaround for 1.0.x versions.

nghttpx: Dynamic DNS

Add Windows CI via AppVeyor

At the moment, we use both resolvers, and if either one is not
DNS_STATUS_IDLE, the other one is also not DNS_STATUS_IDLE.  This may
change if we are going to configure DNS so that either A or AAAA
lookup is done.  In that case, it is better to just return
DNS_STATUS_ERROR in the diff.  This is because the calling side does
not expect DNS_STATUS_IDLE in that case.