StemmerOverrideFilter should not copy the stem override dictionary in it's ctor.

Currently the dictionary is cloned each time the token filter is created which is a serious bottleneck if you use this filter with large dictionaries and can also lead to OOMs if lots of those filters sit in ThreadLocals and new threads are added etc. I think cloning the map should be done in the analyzer (which all of our analyzers do btw. but this is the only TF that does that) no need to really copy that map.