[SPARK-20641][CORE] Add key-value store abstraction and LevelDB implementation.

This change adds an abstraction and LevelDB implementation for a key-value
store that will be used to store UI and SHS data.

The interface is described in KVStore.java (see javadoc). Specifics
of the LevelDB implementation are discussed in the javadocs of both
LevelDB.java and LevelDBTypeInfo.java.

Included also are a few small benchmarks just to get some idea of
latency. Because they're too slow for regular unit test runs, they're
disabled by default.

Tested with the included unit tests, and also as part of the overall feature
implementation (including running SHS with hundreds of apps).

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #17902 from vanzin/shs-ng/M1.