Modules
This hack allows a db tree to keep in RAM a counter of the number of entries
it contains, which is used to call .len() on it. This is usefull only for
the sled backend where .len() otherwise would have to traverse the whole
tree to count items. For sqlite and lmdb, this is mostly useless (but
hopefully not harmfull!). Note that a CountedTree cannot be part of a
transaction.