Move `TAsyncProtocolProcessor` into main thrift library

Currently, when using asynchronous `cob_style` servers with Thrift, linking the `thriftnb` library is mandatory to have the definitions of the class `TAsyncProtocolProcessor`. This implies linking against `libevent` for the Evhttp async. client and server. When using custom transports like ZeroMQ, `libevent` is not necessary and thus, using it should be optional.

The easiest way so solve the issues seems to move `TAsyncProtocolProcessor` to the `thrift` library itself. At least for the server side. I haven't checked the client side yet.