Py-zipkin

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- context_stack will now default to `ThreadLocalStack()` if passed as
`None`

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- Add support for using explicit in-process context storage instead of
using thread_local. This allows you to use py_zipkin in cooperative
multitasking environments e.g. asyncio
- `py_zipkin.thread_local` is now deprecated. Instead use
`py_zipkin.stack.ThreadLocalStack()`
- TraceId and SpanId generation performance improvements.
- 128-bit TraceIds now start with an epoch timestamp to support easy
interop with AWS X-Ray

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- Add batch span sending. Note that spans are now sent in lists.

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- Be defensive about having logging handlers configured to avoid throwing
NullHandler attribute errors

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- Don't log ss and sr annotations when in a client span context
- Add error binary annotation if an exception occurs

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- Fixed server send timing to more accurately reflect when server send
actually occurs.
- Replaced logging_start annotation with logging_end