Record Groupers interface should have a clearFileBuffers method

In the last few months the sink connectors have been updated to use a new way of uploading data that reduces memory held and speeds up the transfer process.

As part of this a new method ` void clearFileBuffers(final String recordKey)` has been added to some Record groupers.

This method should be added to the `RecordGrouper` interface and throw a `NotImplementedException` on the Record Groupers which do not use this method.