Mark AspNetLayoutRendererBase as obsolete since DoAppend gives little value

[snakefoot]

The pre-validation of whether active HttpContext is available provides very little value, but gives a performance hit.

[bakgerman]

Completed this in a branch until ready for merge

[snakefoot]

Thank you for making the hard work. But notice that NLog v6 is still far into the future (maybe a year or more), so your branches might grow stale (reason why issues was not marked as "up-for-graps")