Tchannel

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- Peer selection is now constant time instead of linear time. This should
significantly reduce CPU load per request.
- Fixed a bug where certain errors while reading requests would propagate as
TimeoutErrors.
- Attempting to register endpoints against a synchronous TChannel now logs an
INFO level message.
- Reduced default advertisement interval to 3 minutes.

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- Zipkin traces now include a server-side 'cn' annotation to identify callers.
- Reduced "unconsumed message" warnings to INFO. These are typically generated
when Hyperbahn garbage collects your process due to a timed-out
advertisement.
- Handshake timeouts were incorrectly being surfaced as StreamClosedError but
are now raised as NetworkError.
- Reduced default tracing sample rate from 100% to 1%.

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- Fixed a bug that caused silent failures when a write attempt was made to a
closed connection.
- Reduce ``StreamClosedError`` log noisiness for certain scenarios.
- Make ``TChannel.advertise`` idempotent and thread-safe.

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- Reduce read errors due to clients disconnecting to INFO from ERROR.

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- Fixed an unhelpful stack trace on failed reads.

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- Fixed a logging error on failed reads.