Add support for compressed connection from ProxySQL to backend servers

[aismail]

Why

Depending on the traffic patterns, sometimes it makes sense to enable compressed connection to the backend servers.

What

• mysql_servers.compression is taken into account as the min_compress_length, the smallest package size for which compression will be enabled
• backend connection code enables the compression options for the MariaDB client library when needed

Notes

Will create a follow-up issue for automated Docker test to see that the compression is indeed working. The idea for that is to check the amount of traffic on the network interface used by ProxySQL to communicate with the backend server before and after sending a big package with compression enabled. That big package should be easy to compress - something like REPEAT('a', 1048576).

[renecannao]

Variable min_compress_length needs to be deprecated , and mysql_servers.compression should become a "boolean"

[aismail]

@renecannao why do you want to deprecate it? I recognize the use-case for it was quite subtle :)

[renecannao]

Because it is not configurable in the client library. It is actually hardcoded :
#define MIN_COMPRESS_LENGTH 50