Split insert() into insert() & bulkInsert() (#69)

- Behaviour change for existing insert() users
 - Made the current insert() implementation, as something to use for bulkInsert()
 - Normal inserts now share a lot of code with upsert, which provides benefits like small file handling
 - Refactored/Cleaned up code in HoodieWriteClient for reuse
 - Added a unit test, switching few tests to call bulkInsert() and few to call insert()

hoodie-client/src/main/java/com/uber/hoodie/table/HoodieTable.java

@@ -84,6 +84,17 @@ public abstract class HoodieTable<T extends HoodieRecordPayload> implements Seri
                                                                       Iterator<HoodieRecord<T>> recordIterator,
                                                                       Partitioner partitioner);
 
+    /**
+     * Perform the ultimate IO for a given inserted (RDD) partition
+     *
+     * @param partition
+     * @param recordIterator
+     * @param partitioner
+     */
+    public abstract Iterator<List<WriteStatus>> handleInsertPartition(Integer partition,
+                                                                      Iterator<HoodieRecord<T>> recordIterator,
+                                                                      Partitioner partitioner);
+
 
     public static HoodieTable getHoodieTable(HoodieTableType type,
                                              String commitTime,