Credential-handle retention via missing flag set in Windows Kerberos auth handler

A flaw exists in the server-side SSPI Kerberos authentication handler on Windows Enterprise builds of MongoDB. When processing certain authentication requests, the server fails to properly release an underlying system resource due to an internal tracking error. An unauthenticated user can trigger this by repeatedly sending authentication requests using the Kerberos mechanism, eventually exhausting available resources and causing all subsequent Kerberos authentication attempts to fail. Server restart is required to recover.