Change default behavior for span compression and dropping of spans

[AlexanderWert]

Spec change: https://github.com/elastic/apm/pull/611

This includes:

• [ ] Change default value for span_compression_enabled to true
• [ ] Change default value for span_compression_same_kind_max_duration to 0ms

Agent issues

• [ ] https://github.com/elastic/apm-agent-java/issues/2536
• [ ] https://github.com/elastic/apm-agent-dotnet/issues/1662
• [ ] https://github.com/elastic/apm-agent-go/issues/1229
• [ ] https://github.com/elastic/apm-agent-nodejs/issues/2619
• [ ] https://github.com/elastic/apm-agent-python/issues/1495
• [ ] https://github.com/elastic/apm-agent-php/issues/645