Implement SpanProcessor::force_flush

[frigus02]

Implement force_flush as specified here: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk.md#forceflush

Exports all spans that have not yet been exported to the configured Exporter.

ForceFlush SHOULD provide a way to let the caller know whether it succeeded, failed or timed out.

ForceFlush SHOULD only be called in cases where it is absolutely necessary, such as when using some FaaS providers that may suspend the process after an invocation, but before the Processor exports the completed spans.

ForceFlush SHOULD complete or abort within some timeout. ForceFlush can be implemented as a blocking API or an asynchronous API which notifies the caller via a callback or an event. Language library authors can decide if they want to make the flush timeout configurable.