Fix disableHostLookup flag logic

[RostislavPorohnya]

According to the Contributor comment on #1665 issue:
It seems the behavior was like this since commit 83932d6 that introduced the option. It seems reasonable to me to treat this as a bug since the documentation says that the driver won't try to discover the hosts. Although DisableHostLookup would indeed be better name for a config option like that. And it is not clear to me how users use the DisableInitialHostLookup option, resp. if disabling the lookup always can break someone's program.

Currently, gocql queries system.peers in these cases:

• During Session.init if DisableInitialHostLookup is false.
• When reconnecting control connection.
• When calling Session.AwaitSchemaAgreement explicitly.
• When a schema change query successfully executes.
• When a keyspace change event is received.
• When a node is added or removed from the cluster. (Session.addNewNewNode, Session.handleNewNode, Session.handleRemovedNode)

We would need to disable it in all these cases, not just during reconnection.

Originally posted by @martin-sucha in #1665 (comment)

The flag should disable host lookup procedures in cases mentioned above.