Rename `perflog_compat` to a name implying its functionality

Although when the updated logging was introduced in 4.0 we thought that this would be sort of "compatibility" mode, I think it's still valid and widely used. Therefore, I suggest renaming to something more neutral, such as "perflog_multiline" or "perfvars_multiline", something that implies the actual functionality rather than its compatibility with pre 4.0.