open-telemetry/opentelemetry-specification#370 added
the requirement to have a "force_flush" method in the span processors.

This commit exposes an already existing internal method on the batch span
processor that does exactly the same, it also adds it to the span processor
interface and as a no-op to the simple span processor.

Test that:
- processor works after call to force_flush()
- shutdown() flushes the processor