os/KeyValueDB: reduce malloc/free/string copy count

os/KeyValueDB: reduce malloc/free/string copy count

In RocksDB, LevelDB and Kinetic wrapper code, there is unnecessary
string copying. In particular, split_key makes an extra copy of string
(as a basis for extracted key and/or value), which can be easily omitted.
Also, checking for iterator validity generates std::pair with prefix
and key and checks prefix with the one from pair, wasting memory for
key.

Signed-off-by: Piotr Dałek <piotr.dalek@ts.fujitsu.com>