Introduce ignore_active_span option

[ellisv]

Both Tracer::startSpan() and Tracer::startActiveSpan() in implementation should start a new span as child of active span unless ignore_active_span option is set to true.

[jcchavezs]

@ellisv thanks for this! Could you please add some more information on why this is needed or maybe a link to the spec that describes this?

[ellisv]

@jcchavezs https://github.com/opentracing/specification/blob/master/rfc/scope_manager.md#tracer-changes

[jcchavezs]

Thoughts @yurishkuro @tedsuo @beberlei?

[yurishkuro]

Sgtm