Replace IsAlive with IsClosed

IsAlive is ambiguous because the connection may be dead and we do not
know it. It implies the possibility of a ping. IsClosed is clearer -- it
does not promise the connection is alive only that it hasn't been
closed.

pgxpool/conn.go

@@ -27,7 +27,7 @@ func (c *Conn) Release() {
 	c.res = nil
 
 	now := time.Now()
-	if !conn.IsAlive() || conn.PgConn().TxStatus != 'I' || (now.Sub(res.CreationTime()) > c.p.maxConnLifetime) {
+	if conn.IsClosed() || conn.PgConn().TxStatus != 'I' || (now.Sub(res.CreationTime()) > c.p.maxConnLifetime) {
 		res.Destroy()
 		return
 	}