[SPARK-21671][CORE] Move kvstore to "util" sub-package, add private annotation.

...ain/java/org/apache/spark/util/kvstore/ArrayWrappers.java

...ain/java/org/apache/spark/util/kvstore/InMemoryStore.java

.../src/main/java/org/apache/spark/util/kvstore/KVIndex.java

.../src/main/java/org/apache/spark/util/kvstore/KVStore.java

...n/java/org/apache/spark/util/kvstore/KVStoreIterator.java

...java/org/apache/spark/util/kvstore/KVStoreSerializer.java

.../main/java/org/apache/spark/util/kvstore/KVStoreView.java

...c/main/java/org/apache/spark/util/kvstore/KVTypeInfo.java

.../src/main/java/org/apache/spark/util/kvstore/LevelDB.java

...n/java/org/apache/spark/util/kvstore/LevelDBIterator.java

...n/java/org/apache/spark/util/kvstore/LevelDBTypeInfo.java

.../spark/util/kvstore/UnsupportedStoreVersionException.java

...java/org/apache/spark/util/kvstore/ArrayKeyIndexType.java

...ava/org/apache/spark/util/kvstore/ArrayWrappersSuite.java

.../test/java/org/apache/spark/util/kvstore/CustomType1.java

...t/java/org/apache/spark/util/kvstore/DBIteratorSuite.java

.../org/apache/spark/util/kvstore/InMemoryIteratorSuite.java

...ava/org/apache/spark/util/kvstore/InMemoryStoreSuite.java

.../java/org/apache/spark/util/kvstore/LevelDBBenchmark.java

...a/org/apache/spark/util/kvstore/LevelDBIteratorSuite.java