Jianfeng/node store in hdfs

@jakebiesinger @anbangx @Nan-Zhang @Elmira88 

Hi team,

This PR is to change the ReadHeadSet into a ExternalableSet which can be stored in the HDFS.
I make the genomix-hyracks test passed. I will continue working on the other test cases. I'd like to merge it to master first to let us use this collection in the scaffolding.