During FCV upgrade/downgrade to 5.0 we build indexes on uuid_1_xxxx or ns_1_xxxx. Currently this is done using ShardLocal::createIndexOnConfig, which essentially does a foreground index build because it's holding the config.chunks collection lock for the duration of the build. As a consequence, DDL operations and catalog cache refreshes will queue behind this index build.

This ticket is to change this so that it makes use of background index builds. We could use DBDirectClient createIndex.