[monitor-client] The client does not send the alert from the cache after a network connection is regained

Following the research, and the integration test created in https://tracker.mender.io/browse/MEN-5059 it was discovered that if an alert shows up while the client does not have a connection to the network, the alert is not sent when the device regains it's network connectivity.

The client is supposed to (and does) cache an alert and send it once network access is regained.

Acceptance criteria:

• Debug the client, and make it pass the integration test in https://github.com/mendersoftware/integration/pull/1575/files