Measure `time-to-last-byte` in `netbench`

[csosto-pk]

Problem:

Currently netbench measures time-to-first-byte (conn-time), but that could be misleading sometimes.

Solution:

netbench should add functionality to measure the time-to-last-byte. Having that can show the really impact of a QUIC handshake for various connections.

This does not what s2n-quis sends over the wire or any public APIs.

Requirements / Acceptance Criteria:

Collect time-to-last-byte, last/first byte latency ratio and the time-to-last-byte/total data bytes (no handshake bytes) per connection which can then be graphed with netbench-cli.

Out of scope:

N/A