Multiple DCs: When the best location has failed for their tag, log routers peek multiple copies of their data across the WAN

Log routers pull data across the WAN for their tag. This generally allows only one copy of a mutation to travel across the WAN before being made durable on multiple remote logs.

If the log that was the best location for the log router's tag fails, the log router will peek its data from the rest of the logs to find the data it needs.

It would be more efficient if this combining of multiple streams of data to reconstitute the tag could happen on a single log in the other region, and that log would forward the data to the log router across the WAN.

No file, test, or entry point is named. Start by tracing how log routers peek multiple copies across the WAN when the best location for a tag has failed, then investigate where streams are combined. Done means a single log in the other region reconstitutes the tag's data and forwards it across the WAN.

Written by the indexing model from the issue text.