Extend MapOutputTracker to support tracking and serving the metadata about each merged shuffle partitions for a given shuffle in push-based shuffle scenario

Similar to MapStatus, which tracks the metadata about each map task's shuffle output, we also need to track the metadata about each merged shuffle partition with push-based shuffle. We currently term this as MergeStatus.

Since MergeStatus tracks metadata from the perspective of reducer tasks, it's not efficient to break up the metadata tracked in a MergeStatus and spread it across multiple MapStatus.