Notably, this removes the accidental transient dependency on scala, by removing the dependency on `zipkin-cassandra-core`. Related classes have been vendored into `storage-cassandra`. This also splits out `CollectorSampler` as its own thing and introduces two options for instrumentation sampling: * `BoundaryTraceIdSampler` - For high-volume sites, based on Finagle * `CountingTraceIdSampler` - Trades lower performance for high accuracy