Make MR and Pig writers use the same set of core hadoop classes

[prashantkommireddi]

Pig uses these classes for writing data to phoenix tables https://github.com/forcedotcom/phoenix/tree/master/src/main/java/com/salesforce/phoenix/pig/hadoop

Currently the MR code is isolated, but ideally should use the same hadoop classes for better reuse and maintainability. This will require refactoring these hadoop classes a little, but changes should be minimal.