Add support for 'grave accent' (`) for SET statements parsing

[JavierJF]

• A clear description of the issue

Right now whenever a SET statement is issued containing grave accents like for example:

SET `group_concat_max_len` = 2048

ProxySQL will fail to parse the query resulting into a message like:

7094:unable_to_parse_set_statement(): [WARNING] Unable to parse unknown SET query from client 127.0.0.1:53688. Setting lock_hostgroup. Please report a bug for future enhancements:SET `group_concat_max_len` = 2048

And locking the hostgroup because of it.

• ProxySQL version

v2.2.0

• The steps to reproduce the issue

Issue any set statements in which the variable is surrounded by grave accents. Another example:

SET SESSION `wait_timeout` = 3600

Results in:

2021-06-11 18:24:11 MySQL_Session.cpp:7094:unable_to_parse_set_statement(): [WARNING] Unable to parse unknown SET query from client 127.0.0.1:53692. Setting lock_hostgroup. Please report a bug for future enhancements:SET SESSION `wait_timeout` = 3600