Scram secrets are currently cached in mongoc_cluster_t, which is not available when doing speculative auth for single-threaded clients in the topology scanner. To work around this limitation, scram secrets should no longer be cached in mongoc_cluster_t, but rather mongoc_topology_scanner_t.

This came up during the code review for CDRIVER-3559 (see https://mongodbcr.appspot.com/563660001/diff/561540009/src/libmongoc/src/mongoc/mongoc-cluster.c#newcode1361)