TraverseWithSortStrategy fails to register memory pool listener with heap less than 2GB

TraverseWithSortStrategy sets up its listener on memory pool using a configurable minMemory. But, as noticed by rombert [0] with failure of FlatFileStoreTest#basicTest with less that 2G heap - the class should account for available memory too.

[0]: https://lists.apache.org/thread.html/881e49cca9df85bf506218d6323466b6ad265bd06815194f4ac08e4d@%3Coak-dev.jackrabbit.apache.org%3E