mesh: Save key refresh phase state to node config file

mesh: Save key refresh phase state to node config file

This adds implementation for saving the key refresh phase to
a node configuration file in JSON format. When the key refresh
procedure is finished, the old network keys are remove from the
configuration file.

Change-Id: I33341489ae538cf9734c0a68025865d53f49548f
Signed-off-by: Anupam Roy <anupam.r@samsung.com>