Start pgxpool background health check after initial connections

Otherwise the health check and the create initial connection(s) may both
create connections. While this generally wouldn't be a real problem it
did cause TestPoolBackgroundChecksMinConns to flicker on CI.

pgxpool/pool.go

@@ -222,8 +222,6 @@ func ConnectConfig(ctx context.Context, config *Config) (*Pool, error) {
 		config.MaxConns,
 	)
 
-	go p.backgroundHealthCheck()
-
 	if !config.LazyConnect {
 		if err := p.createIdleResources(ctx, int(p.minConns)); err != nil {
 			// Couldn't create resources for minpool size. Close unhealthy pool.
@@ -240,6 +238,8 @@ func ConnectConfig(ctx context.Context, config *Config) (*Pool, error) {
 		res.Release()
 	}
 
+	go p.backgroundHealthCheck()
+
 	return p, nil
 }