Fix example server's write callback never being called

Handler::wev_ is initialized with a file descriptor set to 0. ev_io_set() is never called for this watcher, so its file descriptor remains 0 and calls to ev_io_start() don't have the desired effect. As a result writecb() is never called.

There may still be an issue due to all endpoints sharing the same watcher, but this is mostly orthogonal to the problem fixed here.

Avoid use-after-free if key update is initiated while ppe is pending

conn_rotate_keys() frees conn->pktns->crypto.tx.ckm. If there is a pending ppe, the ppe object still holds a reference to the freed memory and will eventually try to use it in ngtcp2_ppe_final().

Fix use of uninitialized data in tests

write_single_frame_pkt() returns a packet length greater than the number of bytes it writes to the buffer. The returned length reflects a fake AEAD overhead, while this extra overhead is never written. As a result, ngtcp2_verify_stateless_reset_token() uses initialized data when reading the end of the packet. This causes noise when using a memory sanitizer.

The following options are added:

- --max-data
- --max-stream-data-bidi-local
- --max-stream-data-bidi-remote
- --max-stream-data-uni
- --max-streams-bidi
- --max-streams-uni

Require C++17 for structured binding.

https://tools.ietf.org/html/draft-cheng-iccrg-delivery-rate-estimation-00

Remove unused Stream::recv_data() from server.cc