[SPARK-1768] History server enhancements.

Two improvements to the history server:

- Separate the HTTP handling from history fetching, so that it's easy to add
  new backends later (thinking about SPARK-1537 in the long run)

- Avoid loading all UIs in memory. Do lazy loading instead, keeping a few in
  memory for faster access. This allows the app limit to go away, since holding
  just the listing in memory shouldn't be too expensive unless the user has millions
  of completed apps in the history (at which point I'd expect other issues to arise
  aside from history server memory usage, such as FileSystem.listStatus()
  starting to become ridiculously expensive).

I also fixed a few minor things along the way which aren't really worth mentioning.
I also removed the app's log path from the UI since that information may not even
exist depending on which backend is used (even though there is only one now).
The background thread is a daemon thread, so it will die with the process. There's no need to wait for it to finish what it's doing, since any data it's reading will just be discarded (and it might just hit lots of exceptions because of the shutdown hook ordering).

Unless you'd prefer to register a shutdown hook using Hadoop's ShutdownHookManager to run before HDFS's, but that feels hacky for no real tangible gain (in this case).