Add new JMX metrics.

[Pavel Pereslegin]

Hello, Igniters.

For some monitoring tasks we need to access these metrics through JMX:

1. Total server nodes in cluster.
2. Current topology version.

We can’t access them via JMX for now. Do you mind if I create a ticket
to add them?

I believe, they should be added to TcpDiscoverySpiMBean.

@MXBeanDescription("Server nodes count.")
public int getTotalServerNodes();

@MXBeanDescription("Last topology version.")
public long getTopologyVersion();