Generate payload for multiple datagrams at once

[kazuho]

Up until now, on_send_emit callback has been invoked for each STREAM frame being built. This has become a bottleneck, due to two reasons:

• Applications might have high static cost for generating each payload. For examples, they might be calling pread for each call to on_send_emit.
• Running Accounting and prioritization logic for each packet being built is also expensive.

To mitigate the issuse, this PR refactors the quicly_send_stream function to generate STREAM frames for as much as 10 packets at once.

This PR calls the on_send_emit callback that already exists, and scatters the data being read by calling memmove.

There are two alternatives that we might consider:

• Introduce a new callback to read payload into vectors of vectors (i.e., like readv) that match to the payload section of multiple STREAM frames being generated.
• Let the application provide a pointer to a contiguous temporary buffer that holds data to be sent, and scatter that.

It might turn out that we'd want to try these alternatives, but they require changes to the API. Therefore, as the first cut, we are trying the approach using memmove.