Fix ConcurrentMutationsIT testLockUntilMVCCAdvanced and testRowLockDuringPreBatchMutateWhenIndexed failures on the master branch

These tests are failing because when we set the timestamp in Indexer.preBatchMutateWithExceptions we call KeyValueUtil.ensureKeyValues on the list of cells and then call KeyValue.getTimestampOffset

Pre 2.0 the cells objects are NoTagsKeyValue (which extend KeyValue). In 2.0 they are NoTagsByteBufferKeyValue (which doesn't extend KeyValue) so KeyValueUtil.ensureKeyValues returns a new list of objects on which we set the timestamp.
The fix us to use CellUtil.setTimestamp to the set the timestamp directly on the cell.