In words of keith.bostic

The WT_BOUNDARY structure is really two chunks of information, the information about the disk image we're writing and the information about the pages we wrote to support this reconciliation.

Once the disk image information moves out of the boundary structure, it becomes obvious the remaining information is just a WT_MULTI structure, they're almost identical. My hope is we can create WT_MULTI structures during reconciliation and skip those allocations and that transformation.