Non-thread-safe HashSet used in static methods

see 
https://groups.google.com/forum/?fromgroups=#!searchin/railo/thread$20hang/railo/VyRZWb_nu-k/4WFwZVTWG_YJ

I do not have reproduction steps. I can't verify that it is an issue. I can't consistently reproduce, so I do not know if this fixes it, but I do know two things:

1) HashSet is not thread safe by itself, and it is being used in the two static classes that are in this pull.
2) I haven't ran into an issue yet since applying this change.

There might be a better way to implement this I was going for changing the least amount of code. Feel free to reject and find a better way. As long as it works :)

Thank you!