Reduce reflection overhead in Filter deserialization

Running hbase 2.4.x, I recently profiled one of our clusters which does a very high volume of random reads. An astonishing 12% of CPU time was just spent deserializing in ProtobufUtil.toFilter.

One immediate thought would be to cache String -> Class mappings. Currently Class.forName shows up multiple times (6 in my example) in the profile, each time taking over 1%. I think this is partially due to using FilterList in this example.