[JAVA-2786] Extend health check to all connected nodes

[adutra]

Imported from [JAVA-2786](https://datastax-oss.atlassian.net/browse/JAVA-2786), originally reported by @adutra on Tue, 19 May 2020 16:07:15 GMT.

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As decided internally with other drivers, we should implement health checks following the definition below:

A cluster is healthy from the application standpoint if the driver has at least one open connection to all the nodes not in IGNORED state.

This could be done with session.getMetadata().getNodes() + Node.getOpenConnections() but it’s probably better to wait for [ JAVA-2693 |https://datastax-oss.atlassian.net/browse/JAVA-2693] Resolved .

┆Issue is synchronized with this Jira Task by Unito

[adutra]

Link imported from Jira: this issue (JAVA-2786) --- is blocked by ---> JAVA-2742.

[adutra]

Link imported from Jira: this issue (JAVA-2786) --- relates to ---> JAVA-2693 (issue #49).