Make schema agreement query pull only columns that are used

[dkropachev]

Currently schema agreement logic run select * from system.peers, while only schema_version is used, it creates excessive load on cluster and driver side.

Fixes: #468

[Bouncheck]

See comment in SchemaAgreementChecker.java.

I think TokenMap may be sometimes slightly out of sync. I don't know yet if this is a problem.
Coupling the schema agreement check with token map this way has the consequence that zero token nodes will be considered valid peers in the absence of token map regardless of METADATA_ALLOW_ZERO_TOKEN_PEERS setting. Token map may be absent if METADATA_TOKEN_MAP_ENABLED is set to false.

[dkropachev]

See comment in SchemaAgreementChecker.java.

I think TokenMap may be sometimes slightly out of sync. I don't know yet if this is a problem. Coupling the schema agreement check with token map this way has the consequence that zero token nodes will be considered valid peers in the absence of token map regardless of METADATA_ALLOW_ZERO_TOKEN_PEERS setting. Token map may be absent if METADATA_TOKEN_MAP_ENABLED is set to false.

When token information is not avaialble it includes all peers into schema agreement agreement