Improve ConcurrentCleaner by introducing a buffer

[hmottestad]

ConcurrentCleaner immediately delegates registration to one of the underlying Cleaner instances. Most of the use cases for the ConcurrentCleaner are to create a safety net for users who forget to close a query result. Short lived query results could just be stored in a buffer for a short grace period before registering it with one of the underlying Cleaner instances. The actual registration is fairly costly because it creates a PhantomReference.