[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.
style should be 
```
def fetchCachedFile(
    url: String,
    targetDir: File,
    ...
    useCache: Boolean) {
  ...
}
```