Multiple DCs: Save logs into the DBCoreState before copying data from remote DCs.

This optimization already exists for remote logs, however primary logs will not be added to the DBCoreState before they finish copying all the data from the previous generation. If a lot of data needs to be copied across the WAN, we will have to restart that copy if there is a failure before the copy finishes.

No files, tests, or entry points are named. Start by tracing DBCoreState and the primary and remote log-copy paths, then identify when primary logs are recorded relative to copying the previous generation. Done means a failed copy can resume without repeating already completed WAN transfers.

Written by the indexing model from the issue text.