[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).

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #718 from vanzin/hist-server and squashes the following commits:

53620c9 [Marcelo Vanzin] Add mima exclude, fix scaladoc wording.
c21f8d8 [Marcelo Vanzin] Feedback: formatting, docs.
dd8cc4b [Marcelo Vanzin] Standardize on using spark.history.* configuration.
4da3a52 [Marcelo Vanzin] Remove UI from ApplicationHistoryInfo.
2a7f68d [Marcelo Vanzin] Address review feedback.
4e72c77 [Marcelo Vanzin] Remove comment about ordering.
249bcea [Marcelo Vanzin] Remove offset / count from provider interface.
ca5d320 [Marcelo Vanzin] Remove code that deals with unfinished apps.
6e2432f [Marcelo Vanzin] Second round of feedback.
b2c570a [Marcelo Vanzin] Make class package-private.
4406f61 [Marcelo Vanzin] Cosmetic change to listing header.
e852149 [Marcelo Vanzin] Initialize new app array to expected size.
e8026f4 [Marcelo Vanzin] Review feedback.
49d2fd3 [Marcelo Vanzin] Fix a comment.
91e96ca [Marcelo Vanzin] Fix scalastyle issues.
6fbe0d8 [Marcelo Vanzin] Better handle failures when loading app info.
eee2f5a [Marcelo Vanzin] Ensure server.stop() is called when shutting down.
bda2fa1 [Marcelo Vanzin] Rudimentary paging support for the history UI.
b284478 [Marcelo Vanzin] Separate history server from history backend.