[DO NOT MERGE YET] add options for testing retransmission behavior

[kazuho]

This PR adds two fields to quicly_context_t as well as command line options for controlling them in cli.

• fragment_payload - cause fragmentation of stream data being sent
• destroy_packet - ratio at which packets being sent should be destroyed (so that they would be rejected by the receiver)

These options can be together used as a way of testing retransmission behavior regardless of type of the payload (assuming that the message being transmitted is longer than one byte).

Note: for reproducible behavior, use of x25519 as key exchange is recommended (-x x25519), because the encoded size of secp256r1 public key can change by one byte.

Builds on top of #497.

[kazuho]

FWIW, I'm confident with fragment_payload, but am uncertain if we need to have destroy_packet inside lib/quicly.c.

The latter can instead be implemented inside the code that uses quicly (e.g., quicly/cli, h2o), by discarding datagrams probablistically.