Release 0.30.0

WARNING: this release breaks compatibility with previous versions.

The most disruptive changes are:

1. The module jaeger-core is not bringing Thrift senders anymore. Instead, client applications
   should now depend on the jaeger-thrift module, which will bring the jaeger-core transitively

2. The first step in establishing the public API was made: all classes were moved to an internal package,
   to signal that they are not supposed to be used directly. Classes not in the internal package are part of the
   new public API and compatibility should follow semver. Clients currently using the Tracer.Builder mechanism
   are encouraged to switch over to the Configuration approach. If you face a situation that cannot be done with this
   approach, do let us know.

3. Related to the point above: we now have also a SPI package, intended to be consumed by service providers (components
   implementing Senders, Reporters, Samplers). If you are implementing a service, do let us know! We need your
   feedback before moving forward with this API.

4. Jaeger types related to the OpenTracing standard were renamed, to avoid name clashes: io.jaegertracing.Tracer
   is now JaegerTracer.

Complete list of changes:

• Define some classes internal (#470, @jpkrohling)
• Change return types from Jaeger Span/Tracer/Context to Jaeger types (#469, @jpkrohling)
• Rename Jaeger's Span to JaegerSpan (#454, @jpkrohling)
• Upgrades internals to Zipkin v2 library (#456, @adriancole)
• Make jaeger-thrift's shaded JAR the default one (#461, @jpkrohling)
• Treat gauge.update() parameter as new value, not delta (#463, @mdouaihy)
• Adjust thrift shadow configuration and version (#458, @jpkrohling)
• Remove dependency from jaeger-core to jaeger-thrift (#449, @jpkrohling)