waitUntilDurable() only needs a session to call log_flush() and checkpoint(). Since it uses no cursors, it should be fine to have it simply use one long-lived session object for this, rather than getting a session out of the cache every time.
The benefit for this will be to reduce contention on the mutex that protects the session cache, as it must be locked every time a session is taken out of the cache and every time a session is returned to the cache.