a compression-eligible span must not be buffered on an already ended parent

[trentm]

Spec change: https://github.com/elastic/apm/pull/632

Agent Issues

• [ ] https://github.com/elastic/apm-agent-java/issues/2587
• [x] https://github.com/elastic/apm-agent-dotnet/issues/1686
• [x] https://github.com/elastic/apm-agent-nodejs/issues/2655
• [ ] https://github.com/elastic/apm-agent-python/issues/1533
• [ ] https://github.com/elastic/apm-agent-go/issues/1242
• [ ] https://github.com/elastic/apm-agent-php/issues/672
• [ ] https://github.com/elastic/apm-agent-ruby/issues/1252