cache - copy index before db update.

Added method copy_cache().
cache - protected touched with mutex.

 - void handle::get_touched() test value.
 - bool handle::set_touched(bool) test and set value.
cache - added append.

Splitted merging function in merging and appending.
cache - at end moved from vector to unordered_map.

 - Changed container from vector to unordered map;
 - Removed merge();
 - Renamed append() to add().