Implement support for CLIENT_MULTI_STATEMENTS and multiple resultsets.

[renecannao]

ProxySQL currently doesn't support CLIENT_MULTI_STATEMENTS and multiple resultsets.
Despite the fact that ProxySQL doesn't list CLIENT_MULTI_STATEMENTS as a capability, some application ignore this and send multiple statements in a single COM_QUERY .

Useful links:
https://dev.mysql.com/doc/internals/en/multi-resultset.html
https://dev.mysql.com/doc/internals/en/capability-flags.html#flag-CLIENT_MULTI_RESULTS
https://dev.mysql.com/doc/internals/en/capability-flags.html#flag-CLIENT_MULTI_STATEMENTS
https://dev.mysql.com/doc/internals/en/com-query-response.html#packet-COM_QUERY_Response
https://dev.mysql.com/doc/internals/en/status-flags.html#flag-SERVER_MORE_RESULTS_EXISTS

[renecannao]

Increasing priority on this, as there are 2 bug reports associated to it