Spark+PMEM: long read block time

With the [log](https://gist.github.com/tanghaodong25/00103ce9515b9245a6f40f9f73ba4856), we found remote block fetch time is pretty long. See the benchmark [result](https://docs.google.com/document/d/1JPBagUM-9nt5WeDIvfBiLP4nRgQKxvK3FtutFT5PdM4/edit).
Current conclusion is long read block time issue is caused by Netty. 

