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/test/java/com/uber/hoodie/common/HoodieTestDataGenerator.java

@@ -62,6 +62,9 @@ public class HoodieTestDataGenerator {
             + "{\"name\": \"end_lon\", \"type\": \"double\"},"
             + "{\"name\":\"fare\",\"type\": \"double\"}]}";
 
+    // based on examination of sample file, the schema produces the following per record size
+    public static final int SIZE_PER_RECORD = 50 * 1024;
+
 
     private List<KeyPartition> existingKeysList = new ArrayList<>();
     private static Schema avroSchema = HoodieAvroUtils.addMetadataFields(new Schema.Parser().parse(TRIP_EXAMPLE_SCHEMA));