KVEngine implementations should properly implement isEphemeral()

Storage engines that implement the KVEngine interface (WiredTiger, /dev/null and ephemeralForTest) should properly implement the isEphemeral() function, which indicates whether or not the data will persist to disk.

The default value is false. WiredTiger correctly overrides this function when it changes its behavior, but both the /dev/null and ephemeralForTest storage engines do not, erroneously reporting that they are not ephemeral (and therefore persistent).