fn begin_merge(&self, other: &Self, compaction_frontier: AntichainRef<Self::Time>) -> Self::Merger {
        Self::Merger::new(self, other, compaction_frontier)
    }
Initiates the merging of consecutive batches.

The result of this method can be exercised to eventually produce the same result
that a call to `self.merge(other)` would produce, but it can be done in a measured
fashion. This can help to avoid latency spikes where a large merge needs to happen.
differential-dataflow/src/trace/mod.rs