Implement separate key-value store classes

Change-Id: I156594f546c0231d985dcb60f087f2faa7242848
Reviewed-on: http://naan.mimuw.edu.pl:8001/63
Tested-by: Nebulostore Nebulostore <nebulostore@gmail.com>
Reviewed-by: Szymon Matejczyk <smatejczyk@gmail.com>