Bluetooth: Mesh: Add skeleton for persistent storage

Add initial skeleton for doing settings-based persistent storage for
the mesh network state. This patch only includes restoring some core
network state such as IV Index, Sequence number, Net Keys, App Keys
and the Replay Protection list. The remaining state, and actually
storing the state, is left for follow-up patches.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>