Use a two-file approach for DFSEvaluatorLogOverwriteWriter

A problem that needs to be addressed is that the DFSEvaluatorLogOverwriteWriter currently deletes the old EvaluatorChangesLog prior to writing the new one. Instead, we should use a two-file approach, where an alternative file, EvaluatorChangesLog.alt, is used in tandem with EvaluatorChangesLog.
On read, the newest between EvaluatorChangesLog and EvaluatorChangesLog.alt should always be read.
On write, we always overwrite the older file between EvaluatorChangesLog and EvaluatorChangesLog.alt.