void start(V1Span source) {
    first.clear();
    spans.clear();
    cs = sr = ss = cr = ms = mr = ws = wr = null;
    newBuilder(first, source);
  }
Allows you to split a v1 span when necessary. This can be the case when reading merged
client+server spans from storage or parsing old data formats.

<p>This type isn't thread-safe: it re-uses state to avoid re-allocations in conversion loops.
zipkin/src/main/java/zipkin2/v1/V1SpanConverter.java