@Override
  public int refCnt() {
    return this.refCnt.refCnt();
  }
Check whether have some RPC patch referring this block.<br/>
For {@link IOEngine#usesSharedMemory()} is true(eg.{@link ByteBufferIOEngine}), there're two
case: <br>
1. If current refCnt is greater than 1, there must be at least one referring RPC path; <br>
2. If current refCnt is equal to 1 and the markedAtEvicted is true, the it means backingMap has
released its reference, the remaining reference can only be from RPC path. <br>
We use this check to decide whether we can free the block area: when cached size exceed the
acceptable size, our eviction policy will choose those stale blocks without any RPC reference
and the RPC referred block will be excluded. <br/>
<br/>
For {@link IOEngine#usesSharedMemory()} is false(eg.{@link FileIOEngine}),
{@link BucketEntry#refCnt} is always 1 until it is evicted from {@link BucketCache#backingMap},
so {@link BucketEntry#isRpcRef()} is always return false.
@return true to indicate there're some RPC referring the block.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketEntry.java