trampoline trace hooks from Tracer to a new ChunkTrace class (#230)

Summary:

The idea of the tracer was to be an RAII holder of state for an individual trace because the `CompressionTraceHooks` class could take multiple traces.

Now that a trace contains multiple independent chunk traces, this is the rationale to do the same for `ChunkTrace`. (Or maybe `ChunkTracer`?) This will make it easier to process for visualization as well, since all the stream, graph, and codec IDs will line up, making it easier to make UI optimizations like merging chunks with identical trees or diffing between chunks.

Differential Revision: D87656706

Author: Victor-C-Zhang