This patch implements another possible command: Lookup of keys that
already exist in the tree somewhere. We use a dict as the test tool:
the postcondition verifies that the value obtained from the fractal
btree matches the one we put in the dict earlier.
If we decide to stop the driver, we should not reply back before we
have a clean state with all BTrees closed.
Also, implement lookup of existing keys in the tree.
; Split the writer tests to the writer parts. This is not entirely
correct since they both read and write, but oh, well.
; Provide a simple statem test which opens trees and puts values.
