Tchannel

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- Tornado 4.2 was listed as a requirement but this was corrected to be 4.3
which introduced the locks module.
- Fixed in issue where clients could incorrectly time out when reading large
response bodies. This was due to response fragments being dropped due to
out-of-order writes; writes are now serialized on a per-connection basis.

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- Fixed noisy logging of late responses for requests that timed out locally.

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- Attempting to register endpoints against a synchronous TChannel is now a no-op instead of a crash.

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- The synchronous client will no longer start a thread when the ``TChannel``
instance is initialized. This resolves an issue where an application could
hang indefinitely if it instantiated a synchronous ``TChannel`` at import
time.

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- Fixed a bug in Zipkin instrumentation that would cause CPU spikes due to an
infinite loop during downstream requests.

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- Add support for zipkin trace sampling.
- ``tchannel.TChannel.FALLBACK`` may now be used to register fallback endpoints
which are called for requests with unrecognized endpoints. For more
information, see :ref:`fallback-endpoint`
- Expose ``timeout`` and ``service`` attributes on ``Request`` objects inside
endpoint handlers.
- Disable the retry for all zipkin trace submit.
- Fix Thrift service inheritance bug which caused parent methods to not be
propagated to child services.
- VCR recording should not fail if the destination directory for the cassette
does not exist.
- Fix bug which incorrectly encoded JSON arg scheme headers in the incorrect
format.
- Add support for ``rd`` transport header.
- **BREAKING** - Support unit testing endpoints by calling the handler
functions directly. This is enabled by changing ``tchannel.thrift.register``
to return the registered function unmodified. See Upgrade Guide for more
details.