[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).
For instance,

```
override def getAppUI(logDir): SparkUI = {
  val eventLogInfo = // ... parse log info
  val replayBus = new ReplayListenerBus(...)
  val ui = new SparkUI(...)
  replayBus.replay()
  // ... set ACLs appropriately
  ui
}

private def loadAppInfo(logDir: FileStatus): ApplicationHistoryInfo = {
  val eventLogInfo = // ... parse log info
  val replayBus = new ReplayListenerBus(...)
  replayBus.replay()
  // ... other application history info
  new ApplicationHistoryInfo(...)
}
```

That way you don't have to check if `ui` is null.