add config to limit the maximum handshake message size

At the moment, there is no limit. We buffer up to 16MB of handshake message.

We should add a field in `quicly_context_t`, that defines the maximum size of the handshake message that can be accepted.

Start by locating the definition of quicly_context_t and the handshake-message buffering and acceptance path. Trace where the current 16MB behavior is enforced; done means a configurable context field limits accepted handshake messages without changing the default behavior.

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