Remove support for non-streaming subsegments

There is currently a huge amount of complexity and some performance overhead due to the ability to optionally stream or not stream subsegments - it continues to cause thread safety issues and is very fragile code. Simpler would be to just have `Entity.end()` immediately send an entity without keeping track of the whole segment.

As the OpenTelemetry integrations with X-Ray do not do the segment batching, I think we can consider that a baseline and remove our support for non-streaming from here too.