multiple lua plugin instances doesn't work

If you use multiple instances of the Lua plugin, then we just keep clobbering the first instance rather than making new plugin instances. That's not good.

map / http://imgur.com @plugin=lua.so @pparam=/opt/ats/one.lua @plugin=lua.so @pparam=/opt/ats/two.lua @plugin=lua.so @pparam=/opt/ats/end.lua