Client fixes and optimizations

Release 1.17.14

- [FEATURE] http_client can now collect stats: time to connect, TTFB,
  time to download, requests per seconds, and bandwidth.  See -t flag.

- [BUGFIX] http_client: -B, -K, and -r can be used at the same time.

- [BUGFIX] http_client: do not display cert chain before each request.
  Now this is only done once per connection, if the handshake is
  successful and -a option is given.

- [BUGFIX] Do not wait to dispatch on_close() after reading.  If a
  stream is scheduled to be closed after on_read() callback returns,
  close it immediately instead of waiting until the end of the tick.
  If client creates new request from on_close() event, they will be
  processed in the same tick instead of waiting for the next one.

- [BUGFIX] HEADERS stream is critical: always treat it with highest
  priority.