Initial tperf tool
Initial tperf tool

Summary: Tool for measuring single stream performance of mvfst. Extremely simple right now. After connection is established server sends as much data as possible to the client driven by the `onStreamWriteReady` callback.

Reviewed By: yangchi

Differential Revision: D15537937

fbshipit-source-id: 11e9f27ea213ad8f0c1bac987c2e9a29fac24a7a

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