Optimizer should recognize risk in cardinality estimation

## Enhancement

Combining multiple predicates may exploit single column statistics (NDVs, topN, buckets) and index NDVs - with the combination of selectivities using either an exponential backoff or independence assumption. Neither of the backoff or independence are supported by statistics. It would be beneficial for the optimizer to acknowledge when an estimate was made with such assumptions.

Using the following DDL and insert for test purposes:

```
CREATE TABLE t2 (
a INT PRIMARY KEY,
b int,
c int,
d int,
e int,
Key (b, c, a),
key (e, d, a)
);

set @@cte_max_recursion_depth=10000000;
INSERT INTO t2 (a, b, c, d, e)
SELECT a, mod(a, 1000) AS b, mod(a, 1000) AS c, mod(a, 10000) as d, mod(a, 2) as e
FROM (
WITH RECURSIVE x AS (
SELECT 1 AS a
UNION ALL
SELECT a + 1 AS a
FROM x
WHERE a < 1000000
)
SELECT a
FROM x
) AS subquery;
ANALYZE TABLE t2;

Query examples

tidb> explain select * from t2 where b = 0 and c = 0 and d = 0 and e = 0;
+-------------------------------+---------+-----------+----------------------------+------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------------+---------+-----------+----------------------------+------------------------------------+
| IndexLookUp_11 | 0.00 | root | | |
| ├─IndexRangeScan_8(Build) | 31.45 | cop[tikv] | table:t2, index:b(b, c, a) | range:[0 0,0 0], keep order:false |
| └─Selection_10(Probe) | 0.00 | cop[tikv] | | eq(test.t2.d, 0), eq(test.t2.e, 0) |
| └─TableRowIDScan_9 | 31.45 | cop[tikv] | table:t2 | keep order:false |
+-------------------------------+---------+-----------+----------------------------+------------------------------------+
4 rows in set (0.01 sec)

tidb> select count() from t2 where b = 0 and c = 0;
+----------+
| count() |
+----------+
| 1000 |
+----------+
1 row in set (0.01 sec)

tidb> select count() from t2 where d = 0 and e = 0;
+----------+
| count() |
+----------+
| 100 |
+----------+
tidb> explain select * from t2 use index (e) where b = 0 and c = 0 and d = 0 and e = 0;
+-------------------------------+---------+-----------+----------------------------+------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------------+---------+-----------+----------------------------+------------------------------------+
| IndexLookUp_8 | 0.00 | root | | |
| ├─IndexRangeScan_5(Build) | 70.48 | cop[tikv] | table:t2, index:e(e, d, a) | range:[0 0,0 0], keep order:false |
| └─Selection_7(Probe) | 0.00 | cop[tikv] | | eq(test.t2.b, 0), eq(test.t2.c, 0) |
| └─TableRowIDScan_6 | 70.48 | cop[tikv] | table:t2 | keep order:false |
+-------------------------------+---------+-----------+----------------------------+------------------------------------+
```