Making ExternalSpillableMap generic for any datatype

- Introduced concept of converters to be able to serde generic datatype for SpillableMap
	- Fixed/Added configs to Hoodie Configs
	- Changed HoodieMergeHandle to start using SpillableMap

hoodie-spark/src/main/java/com/uber/hoodie/OverwriteWithLatestAvroPayload.java

@@ -44,6 +44,10 @@ public class OverwriteWithLatestAvroPayload extends BaseAvroPayload implements
     super(record, orderingVal);
   }
 
+  public OverwriteWithLatestAvroPayload(Optional<GenericRecord> record) {
+    this(record.get(), (record1) -> 0); // natural order
+  }
+
   @Override
   public OverwriteWithLatestAvroPayload preCombine(OverwriteWithLatestAvroPayload another) {
     // pick the payload with greatest ordering value