Make schem agreement query pull only columns that are used

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

Author: dkropachev

core/src/main/java/com/datastax/oss/driver/internal/core/metadata/SchemaAgreementChecker.java

@@ -108,7 +108,7 @@ private void sendQueries() {
     } else {
       CompletionStage<AdminResult> localQuery =
           query("SELECT schema_version FROM system.local WHERE key='local'");
-      CompletionStage<AdminResult> peersQuery = query("SELECT * FROM system.peers");
+      CompletionStage<AdminResult> peersQuery = query("SELECT schema_version FROM system.peers");
 
       localQuery
           .thenCombine(peersQuery, this::extractSchemaVersions)