Fix all com_init_db and mysql client related issue with use <dbname>

[harshit-gangal]

Fixes: #6153
Fixes: #3060

mysql -h 127.0.0.1 -P 9727 --database=ks:80-                                              
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 5
Server version: 5.7.9-Vitess (Ubuntu)

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mysql> select * from t3;
+-----+------+------+
| id5 | id6  | id7  |
+-----+------+------+
|   3 |    4 |    3 |
+-----+------+------+
1 row in set (0.00 sec)

mysql> select database();
+------------+
| database() |
+------------+
| ks:80-     |
+------------+
1 row in set (0.00 sec)

mysql> use `ks:-80`;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> select database();
+------------+
| database() |
+------------+
| ks:-80     |
+------------+
1 row in set (0.01 sec)

mysql> select * from t3;
+-----+------+------+
| id5 | id6  | id7  |
+-----+------+------+
|   1 |    2 |    3 |
|   2 |    2 |    3 |
|   4 |    5 |    4 |
+-----+------+------+
3 rows in set (0.01 sec)

mysql> use ks;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> select * from t3;
+-----+------+------+
| id5 | id6  | id7  |
+-----+------+------+
|   1 |    2 |    3 |
|   2 |    2 |    3 |
|   4 |    5 |    4 |
|   3 |    4 |    3 |
+-----+------+------+
4 rows in set (0.03 sec)

mysql> use unknowndb;
ERROR 1049 (42000): vtgate: http://harshit-X1:9725/: Unknown database 'unknowndb' (errno 1049) (sqlstate 42000)
mysql> select database();
+------------+
| database() |
+------------+
| ks         |
+------------+
1 row in set (0.00 sec)

[systay]

👏

[deepthi]

Finally! 🎉