No throttle when throttle_max_bytes_per_second_to_client == 0

[carsonip]

If throttle_max_bytes_per_second_to_client == 0, disable throttling.
This matches the mysql convention of 0 == no limit.

[pondix]

Automated message: PR pending admin approval for build testing

[carsonip]

@renecannao

1. Should we change the default to 0? Although I think the current default is OK.
2. Should we change total_written in https://github.com/sysown/proxysql/pull/1953/files#diff-6535427570d12b480d5c0d5f83c0641eR468 to unsigned long long? It looks possible that this int may overflow. But it will not affect anything if throttle_max_bytes_per_second_to_client is 0.

[renecannao]

@carsonip:

1. I like the idea of changing default to 0
2. I don't think is needed. Because of this , total_written shouldn't be able to exceed INT_MAX/10 + a network buffer. Thoughts?

[carsonip]

2. I don't think is needed. Because of [this](https://github.com/sysown/proxysql/pull/1953/files#diff-6535427570d12b480d5c0d5f83c0641eR496) , `total_written` shouldn't be able to exceed INT_MAX/10 + a network buffer. Thoughts?

Right. It will only overflow when throttling is disabled, but it doesn't matter.

The next thing to do is to update the wiki: https://github.com/sysown/proxysql/wiki/Global-variables#mysql-throttle_max_bytes_per_second_to_client

Also I have changed the default of the variable. Please merge this PR if it looks good to you.

[renecannao]

Thank you!