Extract storage types as JDK 1.8+

[codefromthecrypt]

@anuraaga noticed we could consider making the storage types (and in-memory impl) as JDK 1.8+

https://github.com/openzipkin/zipkin/pull/3216#discussion_r495899024

What would need to stay in the core jar are the following types, as they are needed by zipkin-reporter:

zipkin2.Call
zipkin2.Callback
zipkin2.CheckResult
zipkin2.Component
zipkin2.Endpoint
zipkin2.Span
zipkin2.SpanBytesDecoderDetector << accidentally in wrong package I think!
zipkin2.codec.BytesDecoder
zipkin2.codec.BytesEncoder
zipkin2.codec.Encoding
zipkin2.codec.SpanBytesDecoder
zipkin2.codec.SpanBytesEncoder
zipkin2.v1.V1Annotation
zipkin2.v1.V1BinaryAnnotation
zipkin2.v1.V1Span
zipkin2.v1.V1SpanConverter
zipkin2.v1.V2SpanConverter

Likely we would also need the following for those aggregating dependencies, if no other reason than to keep model types and codec things together.

zipkin2.codec.DependencyLink
zipkin2.codec.DependencyLinkBytesEncoder
zipkin2.codec.DependencyLinkBytesDecoder

plus of course any internal types implied, such as the codec libraries. The main point is focusing on which types remain in the 1.6+ compatible core jar