Add prePocess postProcess and errorProcess to Server Context

I would like to modify TServerEventHandler to include preProcess, postProcess, and errorProcess capabilities.

1. Add preProcess (rename current processContext)
2. Add postProcess so that listeners can perform cleanup after each process (or count number of processes completed)
3. Add errorProcess to report any errors while processing

In particular the errorProcess is helpful, because Thrift includes some internal logic about which error messages are worthy of printing and which are not. This is logic that should be moved to the application layer as I want to be able to make that decision.

https://github.com/apache/thrift/blob/master/lib/java/src/org/apache/thrift/server/TThreadPoolServer.java