Spec compliant SamplingResult support

[legendecas]

According to spec here about span sampling (https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk.md#sampling), there are three flags representing how a span was exported:

1. NOT_RECORD, same as https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-api/src/trace/trace_flags.ts#L22, do nothing with the span.
2. RECORD, record span events but do not export span.
3. RECORD_AND_SAMPLED, record span events, and export span.

As for now, api.Sampler.shouldSample returns a boolean, representing sampling or not. It should return an enum to support all flags in spec.

[legendecas]

Ah.. just noticed a TODO in the code doc. Anyway, I'm going for this.