UnsafeSpan

[codefromthecrypt]

It would be nice to make an unsafe variant of Span, which requires external synchronization and doesn't plan on out-of-band updates. This will make it more efficient to implement layers like Census (#626) who always pass around a span. Also, it will be more efficient for loops or synchronous calls. For example, message tracing or AOP generated spans.

UnsafeSpan.customizer() will likely need to be thread-safe and checked before finish is done. It may be easiest to re-use RealSpanCustomizer for this, then just grab any data before finish.

This layers on #650 but will need some other work, for example, some refactoring about how we share trace-scoped clocks.