private long getPrimaryTimeoutNs() {
    return TableName.isMetaTableName(tableName)
      ? conn.connConf.getPrimaryMetaScanTimeoutNs()
      : conn.connConf.getPrimaryScanTimeoutNs();
  }
The asynchronous client scanner implementation.
<p>
Here we will call OpenScanner first and use the returned scannerId to create a
{@link AsyncScanSingleRegionRpcRetryingCaller} to do the real scan operation. The return value of
{@link AsyncScanSingleRegionRpcRetryingCaller} will tell us whether open a new scanner or finish
scan.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/AsyncClientScanner.java