Tchannel

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- Fixed a bug where Zipkin traces were not being propagated correctly in
services using the ``tchannel.TChannel`` API.

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- Actually fix status code being unset in responses when using the Thrift
scheme.
- Fix request TTLs not being propagated over the wire.

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- Fix bug where status code was not being set correctly on call responses for
application errors when using the Thrift scheme.

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- Make ``TChannel.listen`` thread-safe and idempotent.

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- Fix `retry_limit` in `TChannel.call` not allowing 0 retries.

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- Fixed a bug where the 'not found' handler would incorrectly return
serialization mismatch errors..
- Fixed a bug which prevented VCR support from working with the sync client.
- Fixed a bug in VCR that prevented it from recording requests made by the sync
client, and requests made with ``hostport=None``.
- Made ``client_for`` compatible with ``tchannel.TChannel``.
- Brought back ``tchannel.sync.client_for`` for backwards compatibility.