Apply suggestions from code review

Author: asmyasnikov

options.go

@@ -806,9 +806,9 @@ func withConnPool(pool *conn.Pool) Option {
 	}
 }
 
-func withSharedBalancer(balancer *balancer.Balancer) Option {
+func WithSharedBalancer(parent *Driver) Option {
 	return func(ctx context.Context, c *Driver) error {
-		c.balancer = balancer
+		c.balancer = parent.balancer
 
 		return nil
 	}

with.go

@@ -26,7 +26,7 @@ func (d *Driver) with(ctx context.Context, opts ...Option) (*Driver, uint64, err
 					delete(d.children, id)
 				}),
 				withConnPool(d.pool),
-				withSharedBalancer(d.balancer),
+				WithSharedBalancer(d),
 			),
 			opts...,
 		)...,