Currently all transactions are disallowed on the config database. To allow refineCollectionShardKey to use a single replica set transaction on the config server primary that updates entries in config.collections, config.chunks, and config.tags, this restriction will need to be relaxed to only disallow transactions on config.transactions.

This restriction will not be relaxed on mongos because currently transaction numbers are not propagated by mongos for writes on the config database because of its custom CSRS write retry logic (which will be removed by SERVER-28756).

Proposed implementation:

• Thread a command's full namespace through validateSessionOptions() to CommandHelpers::canUseTransactions() instead of the dbname, replace the check that the database is "config" with a check that the namespace is "config.transactions" if the node's serverGlobalParams.clusterRole is not none.