allow users to clear read buffers

My use case (which I believe many others would share):

• I have a large number of Thrift connections in a connection pool

• Upon return of the used connection to the pool I would like to eagerly clean up the read buffer because I know it will not be used again until the connection is checked out and a new read puts a new buffer in its place.

• Eagerly clearing the read buffers of idle connections saves a considerable amount of heap memory in my application, vs. having all of the idle connections keep all of their read buffers allocated.

• Currently, it looks like someone thought about this and there is a TMemoryInputTransport#clear() , but it isn't callable from people with a TFramedTransport which uses a TMemoryInputTransport for its read buffer (unless you count doing gross and brittle reflection magic).

I've included a patch, but I'm very flexible / open to opinions of how someone more familiar to this project would want to implement a feature like this.