Merge Queue: Optimistic merging and pending failure depth

Merge Queue now supports Optimistic Merging to allow failed tests to be merged if later PRs pass. Merge Queue also now supports Pending Failure Depth to allow failed tests to remain in the queue for…

Merge Queue now supports Optimistic Merging to allow failed tests to be merged if later PRs pass. Merge Queue also now supports Pending Failure Depth to allow failed tests to remain in the queue for longer. Combined these features can increase queue throughput and reduce latency. See Optimistic Merging for more details.