refactor: rename Batch to Partition across pipeline

Rename internal types and channels from Batch to Partition for consistency with the data model.

cmd/go_migrate/metrics.go

@@ -0,0 +1,13 @@
+package main
+
+import "time"
+
+type JobResult struct {
+	JobName    string
+	StartTime  time.Time
+	Duration   time.Duration
+	RowsRead   int64
+	RowsLoaded int64
+	RowsFailed int64
+	Error      error
+}