min-valid-lifetime and max-valid-lifetime not written by config-write

The min/max lifetimes in subnets are missing when the configuration is written using config-write management API command.

Subnet configuration tested and set using config-test and config-set:

{
	"subnet": "192.168.0.0/24",
	...
	"valid-lifetime": 3600,
	"min-valid-lifetime": 3600,
	"max-valid-lifetime": 3600,
}

Subnet configuration written using config-write:

{
	"subnet": "192.168.0.0/24",
	...
	"valid-lifetime": 3600,
}

Kea 1.9.6 on CentOS 7