Keeper monitor status API

[cedricfung]

Now MonitorKeeper sends the keeper node information to the Messenger group for admin monitor manually. But due to the excessive amount of nodes, it's very difficult to identify an offline node.

The monitor could also send the keeper node information to the Safe Observer bot 7000105257 in the same Messenger group. But make a JSON encoded message and the observer bot will loop all messages and update the node state in database.

With the node state in database, the observer could provide an API /nodes to query the network state and display it somewhere on mixin.space.