WL#1467: Improve full_join detection (small table handling)

Current full join detection (and so logging as full query in --log-long-format)
does not handle the case of small tables well - when table is so small what
MySQL decides to do full table scan, even having index choice available. 

Suggestion:  We shall not mark queries as full join if small table is scanned.
The same probably applies to --long-log-format option - it shall not warn if 
first table is scanned, if it is small table.  

We may have variable something like  "log_small_table_size" with default of 10
or so.

Test case: 

mysql> explain select straight_join * from a,b where a.i=b.i;
+-------+------+---------------+------+---------+------+------+-------------+
| table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+-------+------+---------------+------+---------+------+------+-------------+
| a     | ALL  | i             | NULL |    NULL | NULL |    5 |             |
| b     | ALL  | i             | NULL |    NULL | NULL |    3 | Using where |
+-------+------+---------------+------+---------+------+------+-------------+
2 rows in set (0.00 sec)

CREATE TABLE a (
  i int(11) default NULL,
  c char(20) default NULL,
  KEY i (i)
) TYPE=MyISAM;

--
-- Dumping data for table 'a'
--


INSERT INTO a VALUES (1,'a'),(2,'b'),(3,'c'),(3,'g'),(4,'a');

--
-- Table structure for table 'b'
--

CREATE TABLE b (
  i int(11) default NULL,
  c char(20) default NULL,
  KEY i (i)
) TYPE=MyISAM;

--
-- Dumping data for table 'b'
--


INSERT INTO b VALUES (1,'a'),(2,'b'),(3,'c'),(3,'g');