Fixes for 'SetParser::parse1', variable tracking and 'handler_special_queries'

[JavierJF]

• A clear description of the issue

1. Right now there are multiple ways of making the Regex present in 'SetParser::parse1' fail, the original query that triggered this error was:

   SET sql_mode=(SELECT CONCAT(@@sql_mode, ',PIPES_AS_CONCAT,NO_ENGINE_SUBSTITUTION')), inexisting_variable = '+00:00', incorrect_NAMES utf8mb4 COLLATE utf8mb4_unicode_ci;
   

   When executed, ProxySQL will execute an SET sql_mode='' on the backend connection.

2. When part of a SET statement with multiple values fails to be parsed, ProxySQL should try to keep track of the rest of variables tried to be set in it, and then fail.

3. ProxySQL should take into account when locking into a hostgroup has occur for several queries handled by 'handler_special_queries'. Right now this is not the case, preventing the correct execution of several 'SET' statements when locked into a hostgroup.

• ProxySQL version

v2.0.18
v2.1.1

[JavierJF]

Closed by #3318.