Remove custom id_generators and signed hex trace id handling, and make 128bit trace IDs the default

[kawaiwanyelp]

Description

Couple of changes here:

• Makes the get_trace_id function tolerant of any trace ID set in X-B3-TraceId, as trace IDs are supposed to be immutable. Also makes 128-bit trace IDs the default to be generated.
• Removes signed hex trace ID handling. No internal trace IDs being generated are ever '0x' or '-0x' prefixed, nor does the B3 spec allow it.
• Removes the zipkin.trace_id_generator Pyramid setting, as it is only used by YM to make the default trace ID generated the request ID, which doesn't make sense for a trace ID. A newly generated 128-bit trace ID is good enough.

Testing

make test passes

Notes

Following approvals, I'll make a major version bump in master.