fix: set useSystemSchema before policy setup

[dipeshhkc]

During NewSession followed by init,

AddHosts is called in the policy before useSystemSchema is set.

if v, ok := s.policy.(bulkAddHosts); ok {
		v.AddHosts(hosts)
	}

So, getKeyspaceMetadata called by updateReplicas during AddHosts will take useSystemSchema=false and so 2nd query system.schema_keyspaces is called instead of system_schema.keyspaces.

 func getKeyspaceMetadata(session *Session, keyspaceName string) (*KeyspaceMetadata, error) {
	keyspace := &KeyspaceMetadata{Name: keyspaceName}

	if session.useSystemSchema { // Cassandra 3.x+
		const stmt = `
		SELECT durable_writes, replication
		FROM system_schema.keyspaces
		WHERE keyspace_name = ?`
		.......................
        } else {
		const stmt = `
		SELECT durable_writes, strategy_class, strategy_options
		FROM system.schema_keyspaces
		WHERE keyspace_name = ?`	
                .......................
         } 
    	

so was getting following issue

control: error executing "In\t\tSELECT durable_writes, strategy_class, strategy-options\n
It\tFROM system.schema_keyspaces\n\t\tWHERE keyspace_name = ?": table system. schema_keyspa
ces does not exist