Add API for checking network connectivity between all nodes in a cluster

Lack of connectivity from Server to client nodes could be a huge problem for cluster stability. The client will be able to connect to the server and the server will be able to answer to the client as long as the connection established by the client will live, but if it will be closed and the server will try to establish it's own connection, it will just hang(along with an operation, which requires a message to be sent to the client node, it could be topology change or some snapshot stage change, etc).

We should introduce an API, which will be available from control.sh and will allow checking the connectivity between all nodes in the cluster. NOTE that nodes should always try to establish it's own connection instead of just reusing the connection that was established by another node.