Don't use count estimate in postgresql

It produces random numbers that misslead the code.

lib/database/books.go

@@ -87,7 +87,7 @@ func (db *pgDB) getBooks(active bool, query string, length int, start int) (book
 		OrderExpr(order, rankParams...).
 		Offset(start).
 		Limit(length).
-		SelectAndCountEstimate(10000)
+		SelectAndCount()
 	return books, num, err
 }