void setSpan(final Span span) {
    this.span = span;
  }
The {@link ResultScanner} implementation for {@link RawAsyncTableImpl}. It will fetch data
automatically in background and cache it in memory. Typically, the {@link #maxCacheSize} will be
{@code 2 * scan.getMaxResultSize()}.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/AsyncTableResultScanner.java