[db] add typedef for Key (aka std::string)
[db] add new db adapter - avltree. (ARGET, ARSET temporary cmd names).

For now it support only get()/set(), no del() command.
It because we need to delete from std::list too.

The names ARGET, ARSET are temporary names, maybe it will be renamed in
further.
[benchmarks] one_connection: test HDEL, ARGET, ARSET commands too.