Logging should be done with same log level used in condition and log statement

[RalfKoban]

We should report a warning if a log method is invoked inside an if block but does have a different log level than the one checked inside the if block.

E.g.:
Following should report a warning:

if (Log.IsInfoEnabled)
{
    Log.Debug("something");
}

In addition, following should report a warning:

if (Log.IsDebugEnabled)
{
    Log.Info("something");
}