Designed so that if an unexpcted abort() occurs, the bound NetKey remains
in state 2 or 3. If successful, the NetKey is set to Phase 0, and all
bound AppKeys are in their correct state.
This adds APIs to set a directory for storing node's key ring info.
The directory is named MESH_STORAGE_DIR/<node uuid>. This directory
may contain additional node info plus node configuration, if node
configuration storage follows the same layout (as it does when
JSON based config file format is used).
This implements internal key storage add/delete/fetch for the three
basic key types managed in Mesh: Network, Application and Device.
This key storage is separate from keys assigned to nodes within the
mesh, and are used to support Configuration Client functionality.
