"Already cached block XXX" message should be DEBUG

Testing a local cluster that relies on the LruBlockCache for a scan-heavy workload and I'm getting a bunch of log entries at WARN

2018-05-30 12:28:47,192 WARN org.apache.hadoop.hbase.io.hfile.LruBlockCache: Cached an already cached block: df01f5bf6a244f6bb1a626b927377fff_54780812 cb:df01f5bf6a244f6bb1a626b927377fff_54780812. This is harmless and can happen in rare cases (see HBASE-8547)

As the log message notes (and the code confirms) this is a harmless result of contention for getting a block into the CHM. the message should be at DEBUG.