#1 Recover unused space when deleting or replacing objects

[Joseph Huckaby]

Currently (v0.92) DBM::Deep does not automatically reuse freed-
up space when keys/elements are deleted or replaced. The user
has to call the optimize() method occasionally, which is not ideal.
The engine should keep an internal (or external?) table of unused
space for each DB, and reallocate it to new incoming keys/
elements in real-time.