Fixes #41 OOM on bogus thrift input

There are two method calls on the oracle that are exposed through the thrift service. One of them accepts a variable length string for an instance id (which I'm assuming would be somewhere less than 500 bytes) and a 4 byte integer for how many timestamps to increment by. The other does not take an argument. It's not well documented the exact overhead for any headers in the messages but I do know the TCompactProtocol and Thrift in general do a good job at trying to minimize overhead wherever possible. They have some nice reads about the different things they do to preserve as many unused bits as possible.

My conclusion was that 2k just seemed like a decent number that's higher than what I'd expect to be the max message size. This was an effort to save memory and make sure any future bogus messages (and possible DDOS attacks from multiple clients) don't consume too much more memory than absolutely necessary.

Accumulo, on the other hand, has many many methods exposed on different types of services with many variable length string inputs and objects that could reach up to several megabytes in length.