Use compact blocks while doing background validation

We don’t use Compact Blocks while performing background validation if the `assumetxoutset` option was used.

After loading a snapshot and syncing all the way to the chain tip, Compact Blocks can be used for newly announced blocks because the mempool is already being populated even if doing background validation.

Start by tracing the background-validation path used after the assumetxoutset option and compare it with compact-block handling for newly announced blocks. Confirm the mempool-population condition described in the issue, then verify that compact blocks are used after the node reaches the chain tip while background validation continues.

Written by the indexing model from the issue text.