SERVER-42218 changed the default configuration string for normal index cursors from "overwrite=false" to the string "overwrite=false,". The difference is the trailing comma at the end.

The WiredTiger cursor cache has an optimization to skip parsing the config string, but only if it is exactly equal to "overwrite=false". In this case, the additional comma eluded the fast-path and required WiredTiger to parse the config string for every index cursor. This introduced performance regressions on workloads that open many index cursors.

The fix should be to construct a comma-less "overwrite=false" config string for normal index cursors.