Changed header

Change-Id: I2b6370aefa5b0408efc2b510e3acb22dc17072f1
support key prefix

Change-Id: Ib8d34ff8e541af6306a368d4420ab9b56d0af785
Fix key override bug in unset method

Change-Id: I3fc19aa49af6e0e4b575a6900c79ff475067e349
Make lock private

Change-Id: I9b2fd28f1dbcccc8f668805115066247eab15f8c