getRegionLocations needs to be called once in ScannerCallableWithReplicas#call()

Look at this line,
https://github.com/apache/hbase/blob/master/hbase-client/src/main/java/org/apache/hadoop/hbase/client/ScannerCallableWithReplicas.java#L145

It calls getRegionLocations() to get the primary region's locations. It's usage is to figure out table's region replications. Since table's region replication wont be changed until the table is disabled. It is safe to cache this region replication.