Replaces the Mesh model settings_commit callback with a start callback,
indicating that the mesh model behavior is ready to start. Everything
that was previously done in the settings_commit callback may be moved to
this callback, which gets called just after mesh settings are committed,
instead of in the middle of the process.
This resolves an issue where models had no context in which to start
their behavior, as the previous settings_commit call fired before the
mesh was declared valid, making access APIs inaccessible.
Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>
723e14a432