[Discovery] immediately start Master|Node fault detection pinging

After a node joins the clusters, it starts pinging the master to verify it's health. Before, the cluster join request was processed async and we had to give some time to complete. With  #6480 we changed this to wait for the join process to complete on the master. We can therefore start pinging immediately for fast detection of failures. Similar change can be made to the Node fault detection from the master side.

Closes #6706