The application listing is still generated from event logs, but is now stored
in a KVStore instance. By default an in-memory store is used, but a new config
allows setting a local disk path to store the data, in which case a LevelDB
store will be created.

The provider stores things internally using the public REST API types; I believe
this is better going forward since it will make it easier to get rid of the
internal history server API which is mostly redundant at this point.

I also added a finalizer to LevelDBIterator, to make sure that resources are
eventually released. This helps when code iterates but does not exhaust the
iterator, thus not triggering the auto-close code.

HistoryServerSuite was modified to not re-start the history server unnecessarily;
this makes the json validation tests run more quickly.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #18887 from vanzin/SPARK-20642.