Fixed threading bug in ListFormater on threadpool

```[ThreadStatic]``` is not working correctly when using a thread pool. The same behaviour should also occure when using ```async/await``` (but I haven't testet that)

The current behaviour is, that list elements are wrongly concated. e.g:

```{wheres.Count::>0? where |}{wheres:{}| and }``` is converting to ``` where  and col = @col ``` even so there is just a single value in the list.

The provide pull request is using ```CallContext```, which should also work with ```async/await``` related to this blog post: http://blog.stephencleary.com/2013/04/implicit-async-context-asynclocal.html