Make status reporter asynchronous

[mpstefan]

Currently, the NKG status updater is synchronous, which means the status reporter can slow down the event loop.

Consider the following cases:

(a) Sometimes the Gateway will need to update statuses of all resources it handles, which could be ~1000. Making 1000
status API calls sequentially will take time.

(b) k8s API can become slow or even timeout. This will increase every update status API call.
Making updaterImpl asynchronous will prevent it from adding variable delays to the event loop.