Interface for stream protocol with manual buffer control.
Event methods, such as `create_server` and `create_connection`,
accept factories that return protocols that implement this interface.
The idea of BufferedProtocol is that it allows to manually allocate
and control the receive buffer. Event loops can then use the buffer
provided by the protocol to avoid unnecessary data copies. This
can result in noticeable performance improvement for protocols that
receive big amounts of data. Sophisticated protocols can allocate
the buffer only once at creation time.
State machine of calls:
start -> CM [-> GB [-> BU?]]* [-> ER?] -> CL -> end
* CM: connection_made()
* GB: get_buffer()
* BU: buffer_updated()
* ER: eof_received()
* CL: connection_lost()