The relocation function is called whenever it is necessary to move the
data of an item to a different position in the database (e.g. through tree
balancing operations, shifts as a result of adds or deletes, etc.). It is
intended to allow address/position-dependent data items to be stored in a
database in an environment opened with the MDB_FIXEDMAP option.
Currently the relocation feature is unimplemented and setting this
function has no effect.