Discovery: node join requests should be handled at lower priority than master election

When a node is elected as master or receives a join request, we submit a cluster state update task. We should give the node join update task a lower priority than the elect as master to increase the chance it will not be rejected. During master election there is a big chance that these will happen concurrently.

This commit lowers the priority of node joins from IMMEDIATE to URGENT

Closes #7733