[SPARK-2713] Executors of same application in same host should only download files & jars once

If Spark lunched multiple executors in one host for one application, every executor would download it dependent files and jars (if not using local: url) independently. It maybe result in huge latency. In my case, it result in 20 seconds latency to download dependent jars(size about 17M) when I lunched 32 executors in every host(total 4 hosts).

This patch will cache downloaded files and jars for executors to reduce network throughput and download latency. In my case, the latency was reduced from 20 seconds to less than 1 second.
This PR try to avoid network blocking (multiple executors download file by network to local storage only once). So each executor still fetch files to their work directory, but only from local storage ,not from remote node by network.