rocksdb: call spdk_app_stop before exit the process

For some other reason when the host didn't have a block
device e.g: "Nvme0n1", we will call exit(1) directly in
the process, the ASAN will report memory leak issue, so
before exiting the process, we call spdk_app_stop to
do the cleanup.

Fix issue #1422.

Change-Id: I7b7a05807625586bd5b97dc31f41e2b86e1b323f
Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/3586


Community-CI: Mellanox Build Bot
Reviewed-by: JinYu <jin.yu@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>