implement range in Mem_store with a cursor over the map
some accompanying refactoring
add cursors to Simple_store
keep max count in mind in Mem_store.range
remove range from simple_store
move range_entries from simple_store to store
move rev_range_entries from simple_store to store
fix earlier mistakes
jump around
optimize (rev_)range(_entries) operations
Map.cur_first consistent with other cursors
fix failing rev_range quick tests
fix failing base test rev_range_entries
move prefix_keys from simple_store to store
some refactoring in simple_store
more refactoring in simple_store
remove test_backend; fix some behavior and some tests.
cleanup client_protocol
move get_fringe from simple_store to store
cleanup of some unused functions in simple_store + allow next_prefix to handle the empty string
specify the boundary I really mean in get_fringe