plugin/ssa_database.c: Add 'ssa_db_lft' init / delete methods

Moving ssa_db_lft (structure holding CORE layer LFT database),
initialization and deletion into separate routines.

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>