C++ Consolidate client processing loops in TServers

Currently each of TSimpleServer, TThreadedServer, and TThreadPoolServer have their own very similar but not quite identical way of processing a client's lifetime. The code has been copied around and changed; for example a TThreadPoolServer handles TTransportExceptions from process() differently than a TThreadedServer does.

There are certain requirements for this processing loop that needs to be met by every client. Consolidating these three disparate implementations of the client processing loop into one will provide consistency as well as easier maintenance, as there will be one common client processing loop that will contain all the logic from eventHandler->createContext through client->close.

It was also discovered that all three implementations call peek() in each loop which causes more recv calls than are really needed. Will experiment with removing peek entirely; expectation is that it is sufficient to have exception handling around process() and/or have process() return false to end the processing loop, and peek() is likely an unnecessary temporary band-aid that got left there.

This was inspired by changes in THRIFT-2441 and I was encouraged to make this a separate body of work from that change so that it can be reviewed in isolation from other changes.