introducing openTracer without removing old-style tracing

[badiib]

adding a subTracer in the tracer object in channel called openTracer. This means each request needs to have a secondary span object that's called openSpan.

This diff is to define the cut-points for openTracer without removing any parts of the old style tracing.

Still pending:

1. Proper creation of spans given the head k,v map.
2. New tests for thrift and json tracing.

[yurishkuro]

Main suggestion is to minimize the exposure outside of the new methods on the channel, i.e. pass request to them, that way the code outside doesn't even need to know about openSpan field.

[Raynos]

The opentracing library still needs to meet our module requirements. It should be authored in ES5 instead of ES6.

We will also need to benchmark performance.

I recommend you land this onto a "tracing" branch instead of "master" branch.

We used to have a tracing benchmark ( https://github.com/uber/tchannel-node/blob/master/benchmarks/Makefile#L46 ) that may be broken on master / need some parameters tweaked, but it should allow us to verify performance. The benchmark has tracing support ( https://github.com/uber/tchannel-node/blob/master/benchmarks/multi_bench.js#L161-L176 ) and will need tweaking to support writing to jeager / opentracing.

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