isComplete() should not rely on RESULT_COMPLETED

In the course of hammering out #2545672: Handle various migration interruption scenarios → , I noticed that isComplete() (which is meant to identify whether a given migration has completed processing all source items) looks like this:

  public function isComplete() {
    return $this->getMigrationResult() === static::RESULT_COMPLETED;
  }

However, this is not reliable. The migration result represents whether a given migration run as requested completed - a run that was requested to be partial (using --limit or --idlist, say) reports a COMPLETED result, but that does not mean that all source records for the migration have been processed. Also, when performing continuous migrations (as in a daily feed), there may now be new source records available to a dependent migration, and the current migration should not run until those records have been processed.

isComplete() should be implemented as it was in D7 migrate - it should return TRUE iff the number of available source records (source plugin count()) is equal to the number of processed records (id map processedCount()).