Allow multiple SpanProcessors per TracerProvider

[jsuereth]

The Specification states:

• Span processors can be registered directly on SDK TracerProvider and they are invoked in the same order as they were registered.
• Each processor registered on TracerProvider is a start of pipeline that consist of span processor and optional exporter. SDK MUST allow to end each pipeline with individual exporter.

This is a feature request to add a MultiSpanProcessor or AggregateSpanProcessor which can be used to register more than one pipeline with the SDK for processing spans.

An example (abandoned) pr: #590