some refactoring, make the mempool point to a struct that is the key followed
by the leafentry. Next step is to remove the key from the leafentry
Not all tests pass. Known failures in ft are:
ft/test-pick-child-to-flush
ft/ft-serialize-test
ft/test-checkpoint-during-rebalance
ft/upgrade_test_simple
ft/keyrange

Not worth trying to fix these. test_stress1.tdb runs fine. Will focus now on getting rid of
the key in the leafentry