Detect ProxySQL in ShouldGetRealServerDetails

[bgrainger]

A failure to detect ProxySQL may result in connection resets failing or being inefficient; see #874 (comment).

The default server version it uses is 5.5.30; see https://proxysql.com/documentation/global-variables/#mysql-server_version.

Determine whether it's beneficial to get the actual backend server version and thread (e.g., does this break or fix cancellation?) then update ServerSession.ShouldGetRealServerDetails to detect the default ProxySQL version (which seems unlikely to overlap with a real server version in the wild).