forced crash during child device detected may result in duplicate device

If voltha core crashes during childdevicedetected api call by olt adaptor, and the request is retried at a point  after the onu device is added to etcd but before the onu device is enabled by core,  then the onu device will be duplicated when the childdevice detected api is retried.

The device will be in etcd but wont be found in the olt portpeers in voltha, which is used by voltha during childdevicedetected to check if the device exists but it won't be found leading to the creation of a new device